mazikowski

I have a non-turbo '82 280ZX with (I believe) the R180 diff. Whenever I shift (or otherwise abruptly apply power to the rear end) I hear and feel a loud "THUNK". From what I can tell after jacking the back end up, one of the half-shafts seems loose. With the car in gear, if I try turning it, it moves a few degrees in either direction. If I do the same thing with the other one, it does not move at all. Does anyone know what would cause this and, more importantly, what I can do to fix it. I have already drained and replaced the gear lube in the diff.