I pulled off the brake housing and pinion off of the other side and it looks perfect -- shiny as a new silver dollar.  Good.

But the outer race of the main bearing on the pinion is worn pretty bad.  I'm assuming the worn race and the shredded thrust bearing on the other side are connected somehow, but don't know if one caused the other, or if a third problem, like alignment or poor lubrication, caused both of those problems.

Any comments are appreciated.  Has anyone faced this problem?    I obviosly have to replace the main bearing, the thrust bearings, and one of the pinions.   

Also, there weren't any symptoms of these problems while driving the tractor.  Makes you wonder how many 1650s or similar tractors have this problem and we just don't know about it until a larger failure occurs.