I have no idea what you mean by "violent shacking". If you are reporting a loud clunking sound, I would first have the rear transmission mount checked (although the failed mount (clunk) can usually can be heard from coming from under the console. Next, check the front stabilizer link bushings. When they become dry they can also produce a "clunk". Both of these would be more evident when the weight of the vehicle transitions as when turning. Other suspects would be failed A-arm bushings or ball-joints.