i would say it something in the drive train..... universials, axcle CV joints, might be the diff but dont think its coming from there...... you could have the engine stopped, in neutrel, handbrake on, climb under the car and move the tailshaft sideways and look or feel for play in the universals...... maybe take it to mechanic, he can put it on a hoist, he can check it in 5 mins....