I'm posting this in multiple threads for visibility since it seemed like a non trivial amount of GV60 owners had vibration at higher speeds.
For others out there going through this issue, I want to add my abridged story:
My local dealer (Genesis of Madison, really just a Hyundai dealership) was a nightmare to work with, and actually stripped a lug bolt socket while, "fixing," my issue which they ultimately declined to do fully despite 2 independent mechanics agreeing on the issue. Weirdly, they replaced the 1 rear drive shaft under warranty that wasn't in as bad of shape as the other, and rebalanced my wheels. They charged me ~$340 for the rebalance but obviously the issue wasn't gone because the other drive shaft needed replacing, and now I needed a
wheel hub repaired that they said they, "couldn't have damaged." You were the only morons that worked on the car... you did it, a$$holes. Also, I'm going on a limb that rebalancing wheels after this kind of maintenance should/would be related to the warrantied part and therefor covered.
The issue for me, as noted, was 2 defective rear drive shafts. The boots that go over a joint on the drive shafts simply fell apart/slipped off through no fault of my own, all the grease inside was slung out, and the bearings simply did not do their job at this point. There was no damage to anything else on the way to these boots so it wasn't like I ran something over and perished BOTH boots, and nothing else around them. It was simply insane to suggest otherwise. Enter the vibration at higher speeds (for me, 65-75+) virtually all of the time. Before the diagnosis and ACTUAL repair of the drive shafts, I tried 3 sets of wheels with 3 sets of tires (including all OEM). Tires/wheels were all fine, obviously, as it was the drive shafts.
I worked with Genesis corporate to then take my vehicle to Genesis of Appleton who were a DREAM to work with. They had my car for 2 months but I had a loaner the entire time, and they actually fixed everything. It sounded like Genesis corporate would be going after Genesis of Madison for at least some of the repair costs, but $3500 later in parts/labor, my GV60 Performance was fixed and I owed none of that.
Good luck to all but it may be worth $50-150 to have an independent mechanic check your rear drive shafts (maybe front as well), and take that to your Genesis dealer if they don't believe you/won't work with you to replace this obviously defective part on some GV60s.