[Model51]

New bearings by Timpkin are still available for the input and output shafts. There are also two roller bearings on an internal shaft. Those come up NOS now and then and you can also check with the old Ford parts suppliers.

Clean and inspect the gears carefully. These transmissions are normally very bullet proof and reliable. Hopefully you can figure out what happened to yours.
I have some used gear parts if you need something.

Lubrication of the differentials is important and maybe the prior owner overlooked proper maintenance. To inspect and clean the rear differential you will have to drop the driveshaft which you can do without removing the rear axle.

Let me know how things go and I can guide you if you need help.