Chris -  Thanks for the comments and help.  

I bought the tractor 5 years ago and don't know if anyone went into the differential and added or removed shims.  When I was taking the brake housings off, there weren't any apparent scratches or dings on the tabs of the shims from someone pulling them out.   The tractor only had 3,800 hours on it when I bought it.

When I bought the tractor, the rear end oil was pretty cruddy and the oil filter was caved in.  Some oil is channeled into the differential through the left side pinion.  I'm thinking that maybe poor lubrication could have contributed to or caused the small thrust bearing to fail, causing an increase in end play of the differential,  and then wiping the main bearing on the pinion.  At any rate, it all has to be fixed.    

The pinion that I need has a part number 164146AS and has to come from tractors with serial numbers 187546 - 188237 or 188294 - 205158.    The company I got the &quot;worn&quot; pinion from last week is supposed to be checking to see if they have a &quot;good&quot; one.  

I'm also going to follow up on the availability of a pinion from the one on e-bay.

The hydraulic unit is still off the tractor.  Hopefully I don't have too much problem checking and/or adjusting the end play and backlash.  That's covered in my shop manual.  But it sounds like it could be a bit time consuming, which is what this project has turned into......