So I just got back from my dealer visit.
Apparently GM is well aware of the issue. The service manager I worked with was a really cool guy and he said they have had a issues with multiple trucks and have had the engineers out more than once to try and diagnose what the issue is with no luck. He said GM has know about the issue for about 6 months and just recently told the dealer to stop throwing parts at the truck. He said they have tried replacing tires, rims, rear and, and driveshafts on multiple trucks with it never fixing the issue, so essentially they are awaiting GM to figure out whats going on.
As far as my ruck goes, we got it up to 75 twice in a ride along and couldn't replicate the issue. It drove me nuts and made me look liek a idiot, but he said he could try RF balancing the tires if I wanted. I declined for the time being as I wanted to see why it wasn't vibrating now and to see if it still was. On my way home on the highway, sure enough, it started vibrating again at 72mphish. The only difference was the roads we were driving on, one was a "smooth" highway, which made it vibrate, and the other was a "smoothish" back road. I took another way home as opposed to the normal highway route and got up to 75 mph, and no bad vibrations at all...... Does not make any sense besides the highway I am driving on is messed up, but that would have to mean its messed up for a 20 mile stretch on both sides.
I wonder if on the "back roads" if the suspension is hitting smaller bumps that is "loosening" up the suspension making it not so stiff? I do not know....
The service manager said my Sierra was one of the smoothest he has driven. He could have been blowing smoke, Im not sure. We drove a "loaner" Sierra that vibrated WAY worse than mine.