Update on my truck:
As I mentioned above, they had it for a few weeks. I got it back and to my surprise it does not vibrate/shake anymore on the highway (65-80 mph). They replaced the tires, some body mounts, driveshaft, U joints, and steering bushings. However, I've been driving it now for a few days and now it shakes at 45 mph! I haven't even called them to tell them yet because I want to keep my cool.
They did treat me well (only after I told them I researched this vibration problem online and called GM directly). For my inconvenience they installed a bed liner, a tonneau cover, and paid one months payment for me. Now I have to make the dreaded call and tell them that it's still vibrating. I've been driving loaner cars more than my new truck. I wish they would just buy it back at this point. Now it's always going to be in the back of my head. Every little vibration will be magnified 1,000 times because I'm almost expecting it. The bed liner, cover and payment was a nice gesture...I'll give them that. :-/