Perhaps you have a faulty tyre, here in Australia, case separation of a
front or rear tyre can cause unusual vibrations. Also, if your car is
equiped with both front and rear wheel alignment, your rear alignment could
be out of whack. Tyre wise, jack the drivers rear up, block both front
wheels and release your hand brake and turn the rear wheel slowly running
your hand across the tread area and look for irregular wear, then do the
passenger side and do the same, make sure your front wheels are chocked
first, as many accidents can be leathal.

Good luck

Peter