Have you had your rear wheel bearings checked recently? Almost sounds like wheel bearings, or a loose caliper bolt or something.
To check the bearings yourself, bust jack up the rear of the car so the rear wheels are completely off the ground, grab the top and bottom of the tires firmly, and try and wiggle it loose. If there is any play at the rim (not talking about tire flex) then id take it to a shop for a final opinion.