Pop the hood and have someone watch the engine while you shift (with motor running) from forward to reverse a few times. When in forward, and when in reverse, give it a little gas (FOOT FIRMLY ON THE BRAKE, DO NOT RUN OVER YOUR FRIEND). If the engine looks like a buckng bronco, the front motor mount is shot. Examine all the mounts. Once the mounts are examined/fixed, let us know if you still have the problem.
If you need a new front mount, and you are handy, search for "solid mount". The stock mount is fairly expensive. You can make one for $20.
Keep us posted and we can walk you through this.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)