Car vibration at freeway speeds; why?

After an engine conversion, I would suspect that the pinion angle of the rear diff isn't correct. The pinion angle of the differential should be very close to matching the angle of the transmission, but in the opposite direction. So if the rear of the trans is pointing down, then the nose of the pinion should be pointing up at the same angle, or down a degree or two less (for hard launches). Otherwise your u-joints will be binding and causing the car to vibrate with speed. That's why they use constant velocity (CV) joints on FWD cars.

Buy a cheap angle gauge from Horror Freight and check this out in no time.