climb under the vehicle, go to each end of the driveshaft, push up and down on the end of the driveshaft, ie, side to side, not front to back (although, if it does move front to back with you moving it, you definitely have some problems). If you find some sideways/up/down movement on the pumpkin, then your pinion is out of whack and you need to have the rear end rebuilt, if the tranny end of the driveshaft moves sideways/up/down, then you might need a new driveshaft yolk.
I had something similar on my mustang, ended up when I pulled the driveshaft to check things out, the yolk was severely worn on one side like it was spinning out of balance and wearing on one side. I replaced it but the wobble still existed. The actual cause of the wobble was the harmonic balancer....might check it also. There is not a real good way go check it other than if the rubber dampener between the iron outside ring, the thing that has your timing marks on it and the hub that is bolted to your crank. I would say, at least on mustangs, around 80-100k, they tend to start showing signs of needing replacement. If it ends up being that, and you dog it, it can eventually lead to grenading your engine....yep, my stang ripped the entire front of the block out when the crank snapped because of the harmonic balancer being out of alignement with where it was supposed to be...and the fact I dogged the crap out of it all the time....
