Well, I am officially done. After 5.5 months of driving over 15 vehicles and attempts to fix my original Yukon Denali XL I found a truck that doesn't vibrate and has very minimal (acceptable, really) buffeting. I picked it up yesterday. It has 520 miles on it, of which 400+ are mine from lengthy test drives with it. It drives like I would expect them all to drive.
I'm unsure what my plans are moving forward as it relates to this forum and the Facebook group but i do plan on continuing to monitor them and assisting folks, if needed. Suggestion.....if you can go the buy back route, it seems to be the best deal in the end to get your vehicle replaced. They sell every new car thats part of the buyback process at triple net. This means a couple hundred bucks on either side of employee pricing. I got a pretty loaded Yukon Denali for $67k and buy rate on the lease. I got every penny back from my downpayment (first payment, taxes and small cash down) on original vehicle and GM paid all 5 of my lease payments on initial vehicle. I know my experience isn't the norm (and that's another flaw in this entire thing) but outside of time spent dealing with all of this I feel much better about sticking with this truck. Worst case, I can dump the lease through leasetrader.com and walk away.
I wish everyone here the best of luck. Keep hammering your dealer for assistance. Lastly, if you're considering a new purchase and it's between a small dealer and a volume dealer I'd go with the volume dealer. They have many more resources to throw at this. I ALMOST went with small dealer and there is no way they would have gone through all of this for me (and that's not a flaw with them, just a matter of resources).
Good luck!