1999 Cadillac Concours Heavy Vibration seemingly coming fro

This problem is a sparatic problem and isnt occuring all the time. Occasinaly while driving down the road the car will get a heavy vibration in it. This vibration whil grow in intensity and almost fell like theres a flat tire in the rear of the car. The rear tires are good, no broken belts or anything. I had a sneaking suspicion that it could be a prob with my rear pass brakes but dont know if that makes sense.