Installing a new axle shaft, let me know if it improves. I spoke to one of my friends which I'll be taking it in for other service, he's a GM tech I use to work with before I stop turning wrenches. He has had two jobs/ complaint with the vibration issues as well, from the dealer point of view, it's normal but I know and we all know and he knows it's not normal but no solid fix. He even test drove a loaner truck that did the same thing as the customers complaint vehicle. He ordered a new driveshaft under warranty and the new driveshaft did not resolved the issue. Also like I stated above I test drove my little cousins new truck with 200 miles and hers vibrates at well under hard acc. I haven't taken mines in as a complaint yet but also I don't want techs throwing parts at my truck with no luck.