Found the problem!!!
The hydro unit is really a separate part in the drive system and it was not the cause of the tractor getting stuck in gear. The hydro unit, which I never disassembled, is driven by a horizontal shaft that goes through the raduction gears ahead of the differential. One of the reduction gears rotates on this shaft with a caged needle bearing inside. The needle bearing failed and locked the shaft and gear together. Therefor, whenever the input shaft was turned, the tractor wanted to move . This whole drive system, reduction gears, differential, and input shaft is lubricated by grease only. Luckily there was no damage to the shaft or gear. I was able to buy a new bearing locally for $4.45. I almost scraped a 1020 tractor that I was able to fix for less than $5.
I think this bearing could be the source of trouble for others. Tom