WELL THERE IS ALWAYS THE POSSIBILITY OF IT BEING THE DRIVESHAFT, BUT THERE IS THE CHANCE IT IS THE SAME THING THAT WAS WRONG WITH MY '92 LSL. MINE STARTED SHAKING AT ABOUT 60 MPH. AFTER MY RING AND PINION CHEWED EACH OTHER TO PIECES AND I INSPECTED IT, IT TURNED OUT TO BE THE BEARINGS FOR THE DIFFERENTIAL. TO CHECK IT(I STILL RECOMMEND A PROFESSIONAL OPINION) CLIMB UNDER THE CAR AND WIGGLE THE END OF THE CV AT THE TRANNY. LOTS OF PLAY.... PARK IT!!!!