Sounds like you gents have deduced a solution - excellent job. As I have a 2016 Suburban Z71, I have been following this thread since before I procured the beast. I do not have the buffeting, and wonder if it is a combination of 18" wheels and crossbars with surfboard pads on top (disrupt the air flow). I have noticed a lot of vibration on two prior long-term rental Yukons when exceeding 75mph, but nothing on my Z71.
This reminds me of the radiator fan fiasco that was plaguing all 2005 Subaru Outback turbos (OBXT). Within weeks (seemingly), everyone who had a year old OBXT was experiencing their radiator fans running continuously - first start-up on a 30 degree morning and the radiator fan was going full blast. Noone's dealer could find a fix, and I had taken my OBXT in 7 times to get it resolved. Finally, one gentleman's dealership figured out the fix on their own - no help from Subaru. The gentleman put a pdf of the workorder online, and everyone took it to their respective dealers with instructions to just "do this". My OBXT, of course, had a for sale sign on it at that point.