If you wiggle one side do you feel movement in the other side? If so it is a sign that the differential bearings are going bad.
They aren't too bad to change. I just replaced them in the Getrag on my 88GT. I got Timken bearings for $12+ each. Don't bother trying to pull the old ones off. Just remove the bearing cages with a cold chisel then grind through the race until they crack and release.
[This message has been edited by Carrolles (edited 08-15-2013).]