6400 locked up

Need some help on a 1994 John Deere 6400 MFWD with 640 loader with Power Quad. Was using it and it locked up and would not move. let it sit until it was cooled down and it moves again but has a vibration. Took it to the closet Deere dealer who is 200 miles away and they drained the oil and checked the screen. No metal or debris on the screen and cut open the filter and nothing in it. They are going to have to split it and want $5,000 just to look at it. I am considering bringing it home and working on it this fall my self. I have read at least 2 post on the web about pinion bearings failing? Anyone have any information about these or how big of a job to repair it. Wondering if the cab must come off rather than tipping it. It looks as if the pinion is in the range case between the trans and rear axle.

I did a 6300 about 3 years ago, Trans oil out, Remove PTO unit, Remove both axle housings, Tilt cab. P/Q out over L.H. side of chassis
frame, Range and option boxes out over L.H. chassis frame, pinion shaft is main shaft of range box so you are in to strip/rebuild range
box.
Diff bearing quills out of rear trans casing, roll diff out of rear casing and change crownwheel.
Pinion shaft bearings are modified from production. I charged my customer 28.75 hrs to do the job on farm He helped lifting, lining up and
did all the cleaning of parts.
If you can remove cab it will make it easier, tractor gets very unstable with cab tilted and no axle housings fitted to put axle stands
under, chock front axle pivot well to prevent tractor tilting.
The bearing can be assessed by removing PTO unit and looking in, $5,000 to R/R the PTO housing is taking the p****.
The bearing cage on the pinionshaft I think was plastic, I don't remember anything in the screen on his tractor. Be sure to put a magnet in
the screen plug before you refit screen and restart tractor, escaped roller bearings can roll down and when you refit the screen they go up
inside the screen, the trans pump will find them on start up- ask me how I know this!