<VV> Rear wheel play

[AeroNed at aol.com]

I was cleaning the underside of my car for concours at the Roundup this  
weekend (Why do people do this?). I noticed that the rear wheels have some  in/out 
play, about an 1/8 inch. I checked the u-joint straps and everything is  
tight. I can see that the yoke on the diff is what's moving. I pulled the axles  
(they needed to be cleaned) and checked the bolts holding the yokes to the diff 
 and they're tight. So what's loose? side bearings? or is this OK?
 
Ned