Can the high low synchronizer be replaced with the trans in the truck?
Yes.
If you've never done it before, you better talk to someone who has. If you don't have the proper tooling and experience, it might be better to just pull the whole thing. I've never done it, but have asked questions. Main thing you almost have to have is a way to hold up the main shaft when you pull the auxiliary. It's also helpful to have a jig that will hold the auxiliary level and in place on a floor jack or trans jack. If you ask T.S. nicely, he might have some more tips for you.
Some of the FRO's have to be removed from truck, shift bar housing removed and the snap rings removed from shift bar to the rear section. Sometimes if a FRO suddenly won't shift from low to high it is nothing more than the snap rings broke, which can be seen with the top cover { shift bar housing removed}. Any FRO trans or RTLO trans the counter shaft covers have to be removed first and corresponding preload shim kept accounted for and reinstalled right back with counter shaft it came from. Next the counter shafts need to have retainer straps installed with a center bolt screwed into tail of counter shaft and two bolts through strap to counter shaft cover bolt holes. The counter shafts have to be supported tightly {just enough so those still turn} otherwise the shafts can literally fall out and be out of time. Reason is these trans have tapered roller bearings & races. The strap also keeps races from sliding out. The main shaft won't be a issue as long as yoke is left on and kept tight. A jigged mount can be made that attaches to the yoke where the U-Joint straps go for removal/ just have keep it straight, if trans is removed it's not heavy to handle with two people.
Snap rings exploded view page 120. * https://www.eaton.com/content/dam/eaton/products/transmissions/vehicle-transmissions/fr-series-manual/eaton-fuller-hd-fr-fro-transmissions-service-manual-trsm2400-en-us.pdf
Is that ok shimmy?
Hopefully he just pulls the whole box out and takes it in. There used to be a shop in Bismarck that would rebuild auxiliaries, but now everyone just wants to stick a reman in. I had to disassemble and reassemble a 13 sp in college 25 years ago. All I remember is the ones we had were all ball bearings on the counter shafts, and pulling an aux in the truck is a job for someone that knows *** they're doing. That's not me lol.
Thank you guys i appreciate your advice. Years ago i worked on several transmissons but its been a long time ago. I watched a video were the trans was pulled and turned with the rear facing up and i can see why this would be easyer. It looks like the top would have to come off to get the shift bar to the rear hooked up and unhooked. I am sure i could do it if it is removed . I may not do it at all.