I've been having a rather unpleasant high speed vibration in the 70 for the last couple years. I finally rid the car of it once and for all. I thought it was the driveshaft so I put a Denny's in it, still there. I ruled out the rear and transmission since it was only happening under a load. It turns out it was the transmission mount I had in there. It was one of those polyurethane Energy Suspension mounts. After comparing the Poly one to a stock piece, the Energy Suspension piece was 1/2" if not more thicker than the stock mount which in turn threw the pinion angle way off causing the car to vibrate over 65 MPH. I put the stock mount in, vibration gone.