Your vibration may be coming from a worn bush where the front u-joint yoke rides. Once that wears out in will cause vibration and will eat that seal indicated in Dan's post for lunch. Once that happens, it will appear that the center carrier bearing is "leaking", when in fact there is nothing in that bearing to leak out.
The transmission is designed to allow fluid to flow beyond the front u-joint inside the ball to lubricate the u-joint and the yoke bushing. The rear seal is there to keep you from losing all of your transmission fluid.
