Shootin from the hip but I owuld suggest:
Rotor(should only vibrate with brakes on though)
wheel bearings
BUT FIRST-check your tires-I had a broken belt once that did the same thing. Check you air pressure. Jack your tire until its about 1/4" off the ground and put your car in neutral (block the rear end). Then spin the tire in question. Starting with only a 1/4" space will allow you to easily see if the tire diameter is non-conforming. Also-you can rotate your front tires to the rear and compare. Check tire balancing as well.
