I normally don't like to pile on a thread like this but I just have to share my experience in my possible new truck search.
I have been kicking around the idea of getting rid of the truck and getting a Tahoe/Yukon for some time now. In my research for potential problems I have read this thread in pretty much its entirety looking for some pre purchase insight. I figured "how bad can these problems really be"...."maybe guys are just being picky"...."surely the problem isn't that common or pronounced."
After driving a Yukon this weekend with 4 miles on the odometer I will definitely not be taking a chance on one of these units. The buffering and vibe on the highway (seemed to be most pronounced around 64-68 mph) was TERRIBLE. I feel for you guys that have this problem and can without question know it would make me ill to drive more than ten minutes at that speed. Salesman said he didn't notice at first and asked me to jump back on the highway to point it out so I did. Sure enough once at that sweet spot in speed he without a doubt knew there was a problem and agreed something wasn't right. I could see it on his face, "no sale".
Once returning to the dealer the sales manager wanted to talk to me about the possibility of purchasing this unit even after knowing said issue. I politely told him no way...I'm out on this deal. He had all kinds of reasons why the truck buffered like that. The best one he gave me was the wheels locks on the truck caused the issue and that it could be fixed real easy. I smiled and nodded no thank you.
I can't believe GM lets that many units out with such a problem and like I said I genuinely feel for you guys with this issue. I hope they get something figured out for you because this is way too much money for something to drive like that.