Prolly some slight diff yoke wear....need two folks to check it...one handling the tire/wheel, and one watching the yoke movement.

Thanks Joel. I noticed after I posted this that you already answered this question in another post. 

No harm/no foul!
I have found over the years, that when the rear tire/wheel moves from the 12/6 position, but NOT from 9/3, it is almost always play in the yoke itself. If there is play from BOTH positions, it is almost always the spindle bearings, OR the spindle flange nut is loose.
Sometimes, it may depend on how far down the trailing arm drops when you jack it up, too. The best way to check the yokes is to jack it up from under the shock mount just high enough to get the tire off the ground. If you let the trailing arm hang all the way down, it puts the whole suspension in kind of a bind, and makes it a little tougher to feel the play, or gauge how much play there is. I've never seen one that had absolutely NO play in the yokes. There always seem to have some. Gotta have some room to install the snap ring on the end of the yoke. How much is too much? Can't say...I've never seen any specs, but I would guess it would be pretty small. Ya gotta remember that the weight of the car keeps the inner yokes pressed into the diff pretty danged tight, so the only time that play comes into play(no pun intended) is when carving up the curves at a fast pace...too much play can allow the camber to go off, and the car could get squirrely.
I wouldn't rule out a u-joint, either.

Thanks for the info. I will try to get my brother over so we can check it out futher. I put this rear end in about 14 years ago when I replaced the trailing arms. I couldn't tell you if this movement is new or if it has been there for a while.