190 diesel transmission locked up

Running down the road between fields in 8th gear pulling an AC 13 foot disc,  the engien started bogging down, and then both back wheels locked up and skidded down the road. After a few minutes, we were able to pull the tractor backwards, and get it off the road. The next day, checked fluid levels in the transmission, and it was low. Filled it up, and was able to run the tractor fine.  after it warmed up, it started randomly popping out of gear, but jumping right back in going down hill.  Going up hill, it sounds like it has an extra load on the engine, but otherwise seems to be fine.

I'd assume something major it wrong.  The tractor is a 1964 built with serial number 1128, 1900 original hours, and has never had any work done to the engine or transmission or hydraulics. 

Anyone have an idea what I am in for?  Is this going to cost thousands to fix?  Are teh parts still available?  anyone else have this problem or am I "special?"

Ive had that happen before,   sounds to me like one of   the gears has seized on the main shaft.    Luckiley after it cooled it broke loose, that may be why its poping out of gear as the thrust washer are to thin-wont know that until you tear it down.    Usually if its siezed once it will happen again as damage was done the first time.  Could also be the pinion bearings like niele says, thats not good and will wreck a ring gear pretty quick.  The gears in your trans are kinda common only to the first run of 190s,  I have s/n 1207 at home, but it shows a lot of wear and should have went to the salvage yard, but since its a low s/n I bought it.    Good luck.