My T-63 had the same type of symptoms, pinion gear looked good, ring gear was damaged. It would catch one way then rotate back the other way catching again. It would rotate all the way around but would catch on each rotation.
I was thinking the axles are bolted to the frame? I guess you could pull an axle tube off with transaxle attached.
Look at the manual regarding these transaxles, I think there is one side that is the better side to pull off the axle tube to get access to the differential.
Hope you get it figured out!
They are bolted to the frame with U-bolts. Unlike my 990s, which mounts the transmission to the frame leaving the axle tubes more accessible.