I recently had to replace the rear transfer case seal on my 2002 Denali XL. right before the replacement when i hit 50 + mph i can hear a very distictive noise. As the mph increases so does the noise. So i thought once i replace the seal it would stop. i still have the noise at 50 + and I check the seal again for leaks and now the shaft can be moved around which is causing the seal not to seal tightly around the yoke.
vibration at 50 mph on 2002 GMC Yukon XL Denali
If your Yukon has a two piece rear driveshaft, Then check the center support with bearing and all u-joints as they can look fine with the naked eye but be worn out inside and make the drive shaft come out of balance at high speed,