It is something to do with the torque converter shuddering or rear end differential bearing failure. When accelerating from 0 to 20 mph it is fine but it vibrates like running over a speed bump when going from 20 mph to 50 mph. After it shifts into overdrive it stops vibrating. If you let off on the gas pedal when it starts vibrating it will stop but if you mash on it too much it will get louder. Please help me with this problem. Thanks IsraelR