I have the manuals and parts book. When in gear, both forward and reverse, the transmission input shaft from the reverser is turning and when I put my ear to the transaxle case I can hear stuff moving. No change with the differential lock engaged. When the MFWD is selected the output drive shaft for the front drive spins and the front wheels will move the machine. That tells me, I think, that the transmission drive shaft is connecting to the differential drive shaft, then when the MFWD solenoid is energized it is going forward to engage the output gear/drive to the front wheels. I can turn each turn each wheel by hand when up on the stabilizers. Pulled some fluid out through the transaxle drain plug, no metal evident. Other than that it is a beautiful morning her in Alaska.
Very likely the rivets securing the differential ring gear to the carrier have failed. Not common but saw a few, the failure didn’t create much small debris IIRC. We tried replacing the rivets with bolts on one occasion, but did not last. See screenshot from parts catalog below.
Rivets. I guess we will see when we open it up as I am sure that needs to be done. Thank you!