vibration at 50 mph on 2002 GMC Yukon XL Denali

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.

1 answer
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,