How long have you had the tractor? Someone else might have removed shims and overloaded the thrust bearings. Things might have been loose allowing the bearing to get hammered by the thrust. Maybe someone let one wheel spin for a ridiculous amount of time after the tractor got stuck. It could have even been a defective bearing, or some junk in the oil from something unrelated.

As long as the lube oil was up to level, it shouldn't have been the problem. The differential area is lubricated by the oil that it slings up from the reservoir.

Having said that, the two bearing failures are no doubt related. With everything you are going to be changing, you are going to have to take the hydraulic unit off so that you can set the ring gear backlash along with the bearing pre-load.

I have parts from a 1650 that we stripped down. I'll check if I have bull pinions and their condition. There is also a 1650 being parted out on eBay right now, seller brian4898. I have no connection to this seller, just letting you know what is out there.