I also get a vibration, like instant wheel balance type, when I accellerate real hard. As soon as I feather the gas and get back on it disappears. I have solid engine mounts and was wondering if it could be the trans mount. I have MN12's urethane bushings to install, but just wondered if anyone had experienced a similar vibration. Wheel balance & brake rotors at high speed are fine. I would have expected U-joints to be more of an actual knocking sound.?