Having recently sold a '88 750 (old shaky at 55 MPH) and also replaced the upper control arm bushing son my son-in-laws 325ic (shaky two at 45 MPH). I would agree that the control arm bushings are most likely the culprit. If it were my car, i would buy new control arms (that contain both the ball joint and control arm bushing) and replace the old ones. For me, this cured my vibration issues.