I've bought a fire truck as a replacement for my C65 I need hydraulics for the dump body now on the C65. It won't carry tandem weight. I'd estimate body, and load together at 17000. My engine is a Cat 3208Turbo. Try as I might I can't find room for an engine mounted pump. Shaft drive off the front of the engine isn't going to work as the radiator is in the way, and includes coolers for transmission, and engine oil. I'm down to PTO as best choice. I find PTO with married hydraulic pumps of unknown specification. I'm needing at least 5GPM, 2500 LBS. Am I going down a wrong road?
A transmission mounted PTO and pump can easily make that flow at that pressure. I'm building a small hook lift truck now with a 3.86CI pump, up to 2800psi. At WOT, it'll make ~45GPM, although I doubt it gets ran that fast in practice.
There are published specs. for PTO units from a couple manufacturers. I think I can pull Something like 100 HP from a PTO. I need something like 5. There is stuff in the way preventing me from using a driveshaft to power a separate pump. What I'm finding are "Yard mule" units. I believe it was once possible to choose gear ratio, clutch type, and pump specifications. These days these trucks/transmissions are dinosaurs. When I think of yard mules, I imagine the hydraulic fifth wheel. It raises, lowers, engages, and disengages. I'm concerned they may not need much flow, or pressure.
My PTOs have a direct mount pump, this hook lift, the crane, and the grapple truck. Would that not work for your application?
Here's the only picture I have of it, the shift cover and pump mount is off, but the pump comes back to the flange for the end housing of the trans, had to grind it a bit for clearance.
Yes. I'm curious about output. The seller in California will ship to VT. They know nothing about it. Is it a 1 gal. pump at 200 PSI or a10 Gal pump at 2500 PSI. At this point it is a total mystery. Oh, mine is on the passenger side of the tranny.
The pump just makes flow, although it will have a max PSI spec. I didn't realize you had your eye on a pto setup already. There's no way to guess what you're looking at based on the information provided, or maybe I don't understand your question?
Darn it, WillieB. You know better than to post duplicate threads. Couldn't figure out why my post wasn't there, then found out you had two.
Sorry, after posting I checked, nobody had read it, nobody had responded, and I couldn't find it either.
There are not going to be very many pumps that cant lift a dump box on a single axle unless it looks like a tini tiny pump im willing to bet it will do the simple job your asking it to on top of that getting the right housing and gear for your tranny is the most important part pumps can be changed more affordably What trans is it exactly?
I've learned that PTOs can be fast slow and several steps in between. Separated pumps usually have some info. This married unit is described only as coming from a yard mule. I have no understanding of yard mule hydraulics, I hoped someone else did.
Yeah, I know the feeling. I'm not sure what exactly is going on around here, but a LOT of the guys that a guy could expect to respond are nowhere to be found. Some are still logging on once in a while, but not actively posting. Makes me a little sad, I've been following this site since the days of D8Bob and Deas Plant and am disappointed to see this trend. Maybe everyone is just too caught up with work and life......
The nameplate says only MT643. It is 1990 fitted to an Eone fire truck. four speed automatic. As only one wire runs to it, (I guess back up light switch) I'll presume the M stands for mechanical.
https://www.heavytruckparts.net/det...EA-PARKER&model=SPOTTER/YARD-MULE&searchnum=2 How about one like this looks to have a 1/2 inch output line and isnt a tini tiny pump like some of the yard spotters that come up for the price if you had to change the pump you would still be way ahead of buying new the worst case would be the bed running slowly That one fits on the passenger side cable shifted
i believe the MT stands for medium torque as they are the medium series of Allison. the 643 is basically the same as a 653. the 5 stands for the number of speeds ( not counting lock-up ). the 653 has the additional lowspeed planetary assy bolted to the back of the main case and is easily seen at a glance. i would find a truck wrecker and get the complete pto and pump assy etc off a dumptruck. the allisons were quite popular behind gas engines in ford and gm medium duty trucks with dump bodies as well as smaller diesels( ie. cat 3208 ). i think they were also in garbage trucks. the pto off a 653 should also fit a 643 but may or may not need the gear changed.
oh ya, they were more popular in cities than rural areas
Can't you just get in touch with a Chelsea dealer to find out what you need then search for one or just purchase a complete new one? Those Allison autos were pretty popular. At least that way you wouldn't be guessing.
I've been quoted $2600, it may not work. They haven't seen a new Allison 643 in many years. I went ahead and ordered a PTO with pump from CA I'm in VT. A risky thing, I know. I'm getting desperate.
The used Muncie PTO with attached Parker pump arrived today. PTO is in good condition except someone used an acetylene/oxygen torch to cut a mounting bolt. In doing so, they blew a big hole in the cast aluminum flange. I could cry! Parker pump is big, but Parker's website can't find it. I will call Monday, I'm not aware it needs any seals or parts. They will fit I believe. Willie
Cuttin torch mechanics break one thing to “fix” another!
Two things. *** are they using a torch to cut off a 3/8" bolt that would take all of 15 seconds to remove??? Second, who in the hell packages up something that they obviously damaged removing??? I mean, really??
Update: With considerable help I've identified the PTO as a Muncie, and discovered the model #. Some strong solvent has drizzled across the printed nameplate obliterating several characters. DuaneB at Welding web has two decades old info about these transmissions, PTOs, and Hydraulics. PTO will run at near engine speed, is rated at many times the HP I will need. Pump will move 31 GPM at 3600 RPM, 3600RPM, and 3000PSI. It should be a good match. I'll call the seller to address the damage to aluminum case. The flange surface is OK. The surface a mounting bolt should contact is a crater, nothing for the bolt to contact. I'll call. [
The PTO came from California. It is damaged with cutting torch, or plasma. I don't know what to do. The seller will "get back to me" Willie
Maybe a nice heavy washer like cat uses might find enough surface to tighten down on or have the local welder try to add some back in Hopefully seller will refund some of the cost
Flat squarish piece of steel about 3/8 thick with a hole drilled in the middle to bridge the gap and knock the goobers off?
TIG the hole up and then re drill the hole. It’s the long way home, but it should work. Might be a good reason as any to buy a TIG setup, I don’t use mine very often but it is handy to have and I stick weld with it also.
I'm the local aluminum welder. I feel I can replace the missing metal, but how much the distortion will affect the shape of the gearbox, I don't know.
Your just building up the bolt hole. I don't think you will need to worry about distortion.
Weld it up and take a flat file and straighten out the side where it mounts on the transmission. I think you'll be fine. It's certainly not worth sending all the way back to california. You might get by even with a self locking nut with the flat side/ washer made into the nut.
Will it crack at a later date?
I can't foresee it happening.
Thank you. I'll keep you informed.
It kind of stinks that they would send something out like that, all the way from california, I would probably be a little ticked, but it probably isn't worth the hassle of trying to send it back. If he's willing to send you another one for nothing, I wouldn't turn it down, but if it's full price for a different one, I'd run what you have.
I wanted to cry! It was like buying a pig in a poke. I had only the seller stating it was in "good" condition. The truck, and transmission are old enough I can't find this stuff in every salvage yard. It seems these mated PTO/pump combos must match. New was getting very complicated, and 8 times the cost.
The hole repair has maybe been resolved? The nature of welding aluminum is one thing, aluminum castings the next headache sometimes. Was a braze rod able to fill that depression to allow the fastener an opportunity to hold it onto the trans.... Curious. Maybe report on progress when time allows, thanks.
I'm an aluminum welder, but I had the concern that nothing welded is ever exactly the same shape. The seller is LKQ, a big chain of truck salvage yards. The first came from CA, they then sent one from FL. which had been taken apart, bearing, and shift fork replaced in the wrong places, causing some real carnage in the gears, and shafts. Also, the pump had been replaced with one with shorter shaft, and it only partially engaged the half moon key, buggering both keyways. LKO ultimately has said they don't want to deal with me. Our local truck parts company was bought out by Fleetpride. It turns out the cost for new wasn't as bad as I had thought.
Any news on this PTO for the dump? Figuring if maybe so then it would be here. But then again....
As spring slipped away, I got way busy. Now summer is over, I hope for a bit of a lull in my level of work, and can again spare a few days to work on this. I did buy a new PTO/Pump combination, as repeated attempts at finding used in serviceable condition failed. The project includes moving everything at the rear of the frame forward about a foot. Removing rivets has been a nightmare! These are, I presume crushed , and headed hot. They are shaped like a whiskey barrel. Removing the head, and driving them out with a punch is a fail. I've bought a plasma cutter, and will experiment with it, but that is a maybe at best. Thus far best success is drilling all the way through with a small bit, then opening the bore with an acetylene torch. In any case it is a time consuming process.
Thanks for the update on the rivet removal. Does simply heating the rivet with the torch (after the drilled hole) help to drive them out? Will the new fasteners be rivets or some form of specialized frame bolt?
I've had no time to pursue the rivet challenge. I've bought a washing tip for the acetylene torch, and a new plasma cutter. The Hypertherm 45XP has a few months ago introduced a flush cut kit. I've only experimented a bit with it. No, I will use frame bolts. The rivet holes are a bit loose with 1/2" bolts, but the spring experts say they only use 1/2" bolts for replacement in my size truck, and have never had a problem. Winter is coming, and I hope to find a bit of time to work on it.
I would personally drill the holes out to 5/8" or use a metric size(if there is one) that fits the hole with no movement. it is much easier to do it right the first time than try to keep crossmembers tight after they come loose and wallow out the holes. we used to replace the hanger rivets on Hendrickson walking beam suspensions after they broke. bolts would fit too loose so we drilled them out or used a bridge reamer to get a snug bolt hole. whatever size is used ( grade 8) use the thick, hardened flatwashers under the bolt head and the nut(grade 8 stover locknut). red Loctite, torqued and you're good to go.
I like the way you think. I won't be able to use them in every hole. Some won't have clearance to use 5/8" washer head bolts unless I grind away some of the diameter of the head.
It has been a while. The fire truck has sat untouched except to move it to prevent rigor mortise. I was not finding time in warmer weather, it doesn't fit well in the garage. I found a truck mechanic. The frame is shortened, axle moved, new brakes & springs, New PTO, hydraulic pump. Everything sandblasted & painted. Massive fire body & pump gone. New driveshaft. Exhaust in progress! Getting very close to receiving the dump body. I'd like AC. Might get Mrs B to accompany me on an occasional adventure if I had AC. I just can't figure belts to drive a compressor. I'm pondering an electric unit like those used on some farm tractors & railroad maintenance equipment.
I stopped on my way home from work yesterday Temp must have been 90. Took it for an illegal ride. Couldn't keep the rear tires on the ground. Might have worked better with less air in them. Turbo doesn't offer a great deal of boost, but it was very noticeable. Still can't wrap my brain around the 427 gas engine in my old truck being horsepower rated similar to the 10.6 litre turbo diesel. The Allison transmission wouldn't have been my choice, but it efficiently sends power to the wheels. New springs, new brakelines&tanks Waiting on a new hydraulic valve, (I hope Monday) Then the 14' flat dump goes on.
I've driven similar fire trucks with the 3208 and the 427, and I would take the 3208 any day. I don't think the turbo added much of any HP to the engine. Older ones were available with almost the same HP ratings non-turbo. I think it was added to clean up some of the filth. 3208s were a dirty engine.
i have had several 3208's over the years and the highest power in a non-turbo that i had was a 3208 at 210. the turbo engines were 225 and then 240 and 250 that i saw. depends on the arrangement number. the turbo engines definitely had a lot more power at a lower rpm than a natural topped out at. the natural 210 was rated at 2900rpm and the turbo 240 was something like 2600. the 3208 turbo at 250 would run circles around a 3116 at 250. i was told by cat that there was no comparison because of the tighter emission controls on the 3116. one of the places i worked at had them as plow trucks with slide-in box sanders.
I do remember the non-turbo could be as high as 225, and I think maybe even higher. I may also be confusing that with an 1160, don't remember. These were all highway motors. I did kind of forget about the high RPM, you are right about that. Some of these things were worth forgetting. The later model 3208 turbo motors had a lot of low end grunt for sure. The non-turbo made you want to drive it like the old Detroit Diesel adage.
Research says these engines were lots of different horsepower ratings. It isn't here, or I'd look. It is printed on the engine the horsepower. I remember it as 250. In any event it's more robust than my 427 in the old truck. The 427 tall block was rated all over the place also. I think they rarely were rated at anything beyond 4000 RPM explaining in part why a modern 7 litre is rated at 6000 RPM Yeah that's more horsepower, but running at 6000 makes for a short life.
OK, the truck has been a few runs on road. Tires were OK, but rears were chewed some from tire chains. The old truck has new tires all around, and they are heavier load rating than the fire truck. I wanted to switch tires. The tire dealer is at least second generation, a good guy with enough employees that riding herd on them is more than a full time job. He said breaking down these tires is a big deal, he alone is qualified to do it. Not an option to do them during business hours. He'd try to do one a night. To me that seemed a very big thing, long wait. I decided to break down the fire truck tires. That proved a bigger job than I had imagined. The concept is simple: Free the rubber tire from the bead of the wheel, it falls away a bit from the narrow ring of the wheel. Work the narrow ring free from the snap ring, the snap ring comes easily. In practice this does NOT work! A big pucker of rust formed on the inside of each wheel it is impossible to force a 14 ply tire bead past. You Tube videos show a tire hammer being struck one handed with a soft surface dead blow hammer to free it from the wheel flange. I didn't have either, but noticed it is shaped exactly like a backhoe tooth. I used a 12" wide backhoe bucket with only one tooth. If it weren't for the big swell of rust it'd work. No amount of lube or force was going to push it over that ridge. I dismounted two tires successfully, then ruined one. I finally gave up, and ordered 4 wheels & tires 22.5" They arrived Wednesday, but didn't fit the truck. Still waiting for the new ones.
Next chore: Servicing the transmission. I can't find much on the Allison MT643. What I find is about newer models. Some say older models were to use 15W-40 engine oil, less old used Dexron III, and all can now use synthetic auto transmission oil. They go on to caution that a thorough wash down is essential as synthetic & petroleum oils shouldn't be mixed. Newer models have filters inside the transmission, this has a spin on filter outside. I've seen some mention of an internal filter, and a suction screen. Does anybody know?
Call Weller Reman, or another Allison Authorized Remanufacturer. I have an MT653 in my dump truck and was advised by them to use full synthetic transmission fluid only. Also, I only have an external, spin on filter, no internal filters.
The old Allison MTs have a filter in the pan much like a car transmission. Most are paper and need to be changed but not very often. Some have a steel screen instead and a filter installed remotely on a cooler hose or something. They can run engine oil or transmission fluid but not to be mixed. Just look at what is in there now, and drain and fill. No reason to go to a lot of work to fully flush if nothing is wrong with it. I would not spend for synthetic as the transmission will probably never see the kind of miles to make it worthwhile. They mostly used engine oil in the ones that had a retarder braking system on the transmission. They usually had ATF.
I don't know about a retarder. It locks up to effectively use the engine for downhill holdback. Is this a retarder? Is it enough to drain & clean out the pan, then replace with Dexron III?
A retarder is a normally empty cavity that has some moving blades and some stationary blades attached to the drive shaft or input shaft depending on model. When you want to slow the truck down the transmission fills it with oil which picks up a lot of heat. They were often controlled by a third pedal on units of that era. They are a big chunk on the transmission that would not otherwise be there. There would be an "R" in the model number for retarder. Drain and remove the pan, change the filter, refill. Good enough.
Thanks for that info. Now I track down the external & internal filter & gasket Dexron I'm sure I can buy.
I got nothing to say.
Other threads I've cluttered with my story. I scheduled an inspection last week. I failed on SO MANY counts! All four air cans failed. Something undetectable in right wiper. U bolts on rear springs not long enough, water in headlights, I have new tires & wheels rear, but right front scalloped. I can't see or feel it! My triangles & two fire extinguishers weren't bolted. Air leak when parking brake is applied. Oh well! I got frustrated this morning, Texted Rico, a truck mechanic who does NOT want publicity. I said I fear I'll die of old age before I get a sticker on the windshield. He took over. He says he has an appointment for 7:00 AM tomorrow. He will take it, answer to any challenges. He is confident it will now pass. I'll tell you the outcome tomorrow.
Great to hear. Good Luck!
I just picked it up. It PASSED!!
So did he have to fix much or were they just raking you over the coals on little stuff where you live are they state run inspections or private? Its all done in private shops here and can vary by who is working that day
Privately owned stations. Vermont AOT & State police look the other way when a beater mini van, smoke billowing out of both front windows, six kids under age 8 crawling around inside like a litter of puppies, three donut spares, no working tail lights etc. etc. A truck on the other hand gets flagged out of service for very minor infractions. I heard yesterday of a driver hauling blacktop being pulled out of rotation for a roadside inspection. They fined him because driver's side washer jet hit too low on the windshield. Recent laws: they have to photograph the truck inside the garage for inspection & they have to document any issues if it fails, send that to the State AOT. Next inspector has to sign off on each fault.
I don't object to being forced to attend to issues. My brake cans were functional. The front ones looked rusty from outside, Rear ones, I can't say why they weren't OK. When you set the parking brake on an air brake system all air is relieved from spring brakes. For a time there will be air bleeding off sound. I had been wondering how pressure was bleeding off, but couldn't find a leak. All air lines, tanks, and fittings are new. I guess the spring brake relay valve needed replacement, now it holds pressure much longer.
Latest issue: A substantial shake. It seems to begin at low speed acceleration, or gentle uphill, or cresting the top of a steeper hill heading down. Rarely do I feel it in the steering wheel. It isn't a shimmy one would feel mostly in the steering wheel, this is the whole truck. Driveshaft has been partially replaced due to elimination of the fire pump. All six tires & wheels have been replaced & balanced. All brake cans are new. King pins, steering linkage, and all the usual wear parts have been checked. Odometer reads 32000, but 609 hours of pumping ran those miles up. The odometer runs when the transmission drives the pump. Likely rolling miles under 10,000. I've got a nagging suspicion the driveshaft is involved. 33 years ago I bought my 1956 Power Wagon. It behaved this way, but worse. It had driveshaft clocked 90 degrees out.
i would suggest to check u-joint phasing first and then driveline angles
Exhaustive research, and talking with several people I hoped would know, I get opposite facts. I've tried it both ways. I can't say I can tell the difference.
Can you post some pictures from under and from the side as much as possible? Also check to see if there are wedge shaped shims between the rear axle and the springs. Fire trucks are loaded all the time and sometimes they get creative to compensate for sagging springs that change the ride height. From the look of the very first picture it appears that the back of the truck was low before you removed the fire apparatus so maybe they had done something to change the rear u-joint angle. Just a WAG
The springs were beat, so in the process of shortening the frame it got new springs. I had choices of spring capacity. I chose 22000 rears. There are now, or before no wedges, no intentional changes in pinion angle. I can't get a straight section to gauge. I feel pinion & transmission angle are close, if not matched. They are nearly on plane, the U joints have very slight angularity. I'll get pictures tomorrow. I've been thinking how I can show all drive shaft components & their relation.
There isn’t a bottomed out slip yoke in there any where is there or one pulled almost to the end of the splines
When you removed the fire pump did you add a steady bearing or put in a longer driveshaft? When you took out the fire pump, its like taking out a short driveshaft, so most often, on a longer wheelbase, you would add a steady bearing. On a real short wheelbase you may get by with a single, longer driveshaft
You may well need to go down and dirty on this one, jack up the rear end and sit the axle on some good blocking with the tires up barely off the ground. Run through the gears and see if you can duplicate the shake with a helper. I've found some vibrations from imbalanced driveshafts this way.
View attachment 224828 View attachment 224828 I'm afraid I made quite a mess of posting pictures. All want to post sideways. Repeated attempts at editing I can't delete the sideways ones. Yolk at trans, rear half of middle U joint & differential are all oriented the same. Until yesterday the rear was 90 degrees different. No change in oscillation by changing orientation. As I understand, angularity increases oscillation. There is no angle at center, little at either end.
Now I have something new to be frustrated with. I can't post pictures.
The carrier bearing was already there, a foot behind the pump. There was a short shaft with two U joints & telescopic spline between Trans. and pump. Behind the pump, another short two U joint shaft with carrier, then a longer shaft to rear. Axle has been moved about a foot forward, and a new shaft replaced the two short ones & the pump.
Could there be a problem inside the axle ? Could you remove the driveshaft and tow it? Have you jacked it up and run it?
So how long is your driveline, is it one piece from the trans, all the way to the rear axle? It should be two, one from the trans to the carrier, and then another one from the carrier to the rear. It shouldn't be just one.
Did you mean yolk, or yoke? Never mind, I'm just egging you on.
I'll fix that in future.
It is three universals. A yoke at transmission, and another at differential. Two sections of shaft. Front is tube, I'd guess 4" diameter, approximately 4' long. the carrier bearing is at rear end of this. Rear section is splined, about 3' long. That is next step, to jack it up & run at highway speed. At low speed there is almost no run out. Shaft is not balanced, but at 2000 RPM I feel it'd take a lot of out of balance to cause this much shake. I went to the fire house yesterday. The truck most similar in size is the rescue truck. It has the same configuration but smaller diameter & U joints. Each segment of the shaft is phased the same each end, like mine was before I turned it at the spline 90 degrees. Everything I find on the internet shows a two universal driveshaft. Middle section is always phased the same. Yokes at each end are the same angle. Nobody addresses what it should be for a three universal shaft. Oscillation is greater in sharply angled joints. None of the three in my shaft have more than a few degrees of angle. The middle has none. My logic has been that the middle joint is not contributing oscillation. Like a straight two U joint shaft, as in a pickup, both ends of the shaft should be phased the same. I rode as a passenger in my son's GMC 2500 towing a 25' long deck over trailer empty yesterday. I was thinking this rides similar to my bigger truck empty. I'm going to try letting some pressure out of the tires.
Which shaft isn't balanced? You made the shaft yourself after removing the pump? And yes, the truck will ride rough with no bed no weight I used to work at a place that had a big chevy like yours, we pulled a heavy gooseneck. It would toss you all over without the trailer.
No, I had an engine rebuilder build the front section of shaft. I asked about balance. He said he takes great care to keep everything balanced, and has never had a balance issue. This shop has rebuilt several engines for me, I have a lot of confidence in them.
I cant remember how many drive shafts i built over the years and nun were balanced. I have seen some realy messed up home made drive shafts that ran smooth. We had a truck with a problem like yours it had a bad pinion bearing.
Put the shaft in late afternoon. Sitting not level I got a transmission shaft measurement of 4 degrees, a front shaft of 2 degrees. It is not conclusive the pinion angle, I think I'm off by a degree. I'll study more on it tomorrow. It runs far more smoothly!
Scratch that. Downhill it is bad as ever. I say downhill, I don't mean steep. Today I went to Danby Four Corners 1000 feet higher in elevation than home. Coming down four miles the shake is worse than ever. It's driving me crazy! I've spent a fortune, now it seems nobody has a suggestion.
Tires? I thought you said you put on new rears, but not new steers. I'm just throwing out suggestions. Its frustrating when things don't go right. Does it seem to come and go with throttle applied vs. coasting? Have you checked the nut on the yoke on the rear of the trans? My allison in one of the cranes had a vibration and the trans yoke nut had worked loose. That's the problem with used trucks, you don't know if this is something you caused in shortening, or if its always been like this. How's the steering joints/ kingpins etc? Everything good and tight up front?
Thus far: New wheels (6) new tires (6) brakes checked approximately 10,000 highway miles total. New brake cans. Steering checked, driveshaft worked over by Charlebois 100 miles away, said to be the best. New universals93) new carrier bearing. Charlebois technician showed me a video. Total run out at front yoke at transmission .058 (.029 off center) corrected & balanced. 6 tires balanced. We jacked it up, blocked it up, ran it at 60 MPH & less, far less shake. Braking doesn't seem to aggravate it. All new brake cans, new hoses, Steering doesn't seem to aggravate it. It is most pronounced on gentle grades up & especially down. Seat shakes constantly Much worse sometimes. I look at my foot on the accelerator It is shaking constantly. My pant led actually flaps. A bottle of water on the dash is crashing around. I feel it mostly in the seat. Coming down hill today, I kept noticing a strange flutter noise. It was the flapper on the stack exhaust fluttering with the shake of the whole truck.
Is the engine out of balance (flywheel, crank?) or something in the transmission (torque convertor?) out of balance? I'm no truck mechanic, but I have dealt with finding various vibrations in equipment and large rotating shafts. It can be a pain tracking them down when all the obvious components have been inspected or replaced and very frustrating. Did you get to drive the truck much before you did a bunch of work to convert it to your dump truck? I remember it was an old firetruck, did it have a vibration back then? Also, when discussing natural harmonics, increasing the weight (mass) of an object changes its natural resonance and can get rid of a vibration at the desired rpm range. So, if your driveshaft is too light for the specific design length, it might shift the vibration to a different frequency if the driveshaft is slightly heavier. Did anyone ever chuck the driveshaft up in a lathe and check runout along the shaft at various points to prove that it isn't slightly bent? That would certainly affect its natural harmonic as well. Just some thoughts.
I drove it as a fire truck maybe 70 miles total. Seth drove it another 50. We noticed none of the shake it now has. I believe the shake developed after alterations. We shortened the frame roughly a foot. The fire pump & the fire body were removed. Because the pump served as a sort of transfer case, it was inline of the driveshaft. A short shaft two universals & slip joint ran transmission to pump. A shaft ran through the pump, mated to another U joint, a short shaft reached the carrier bearing, then another u joint, then another slip spline. It ended at the pinion with the last U joint. New drive shaft has U joint at rear of trans, a carrier bearing, another u joint, a spline, at pinion another U joint. Present shaft is three U joints a carrier bearing, and one slip joint. The foremost driveshaft shop replaced three U joints, a carrier bearing, cut the yoke off & corrected out of center. They then balanced the whole assembly. Trying to correct the shake the whole truck got new wheels & tires. The 10,000 mile brake drums have been checked for run out. Tires have been back for rebalance. Rear spring equipment down to bolts is 100% new. Spicer says U joint at transmission needs at least 1 degree, not more than 1-1/2 Degree angle. The angle of front section of shaft must match pinion. None of this seems to be more than a degree out of spec. New angle gauge is ordered.