I have a sim prob, mine is at 65 I get a vib and I can feel it in the seat, Iv'e been told it's the axle bearing.In your case it could be that or a bunch of other things. Possibly the trans. mount could be broken or the bushing is bad. The tourqe arm could be bent, or the bolts that bolt it to the rear could be loose. Tie rods, or bent axle, bent rim, bad tire, low press in the tire, flat spot on the tire. If you feel it in the seat more than likely it's in the back. If the wheel shakes more than likely it's in the front. Hope this doesn't break your spirts, lol, good luck.