Nightfall, I just looked through some documentation and found a diagnosis guideline for vibrations in the driveline/transmission which should help in pinning down what you and others are experienceing. You may want to check in with your dealership and see if you can reproduce the vibration for them and see if they know of this document. Unfortunately I can't get any more specific than "this document". I hope that helps you.
Jeff Morris, Chevrolet Customer Service