When the vehicle gets up to 45 or 50 mph, the steering wheel sometimes begins to shake. Normally I would suspect wheel balance, but I've checked that. In addition, if I do not slow the vehicle down, the shaking of the steering wheel can get even worse until the entire front end starts shaking violently. I've already replaced the rocker arm bushings and the other bushings and related steering components but that has not helped. Any ideas?