We had the same problem with an 88 we did about 2 years ago.  We ended up replacing the front bearing on the transmission input shaft, made a world of difference. 

The old one turned smoothly, and we could feel a little looseness. When I found I had a bearing on hand, it was pretty obvious how loose the old one was.

Also, make sure the set screw in the rear of the coupler has engaged the hole in the input shaft, and is not pushing against the splines, holding it off center.