Virbation, Shutter, Jiggle. 1999 Yukon 4x4.

Hello all!
I have recently purchased a 1999 GMC Yukon 4X4 and notice a jiggle I can't get rid of.
65-75 mph is most noticable and almost non-existant at any other speed.
Feels like a wheel(s) is out of balance. If you can believe it, it actually gets 10 times worse if I roll the back windows down.
In an attempt to remedy the bounce, I have new tires (Big-O Bigfoot A/T LT265/75R16) on stock rims. New shocks. The tires have been balanced 3 times, the most recent on a Load Force Balancer. This actually seems to have made it a tad better. Alignment is fresh and the Idler Arm was just replaced.
I have seen some info regarding motor mounts and possible drive shaft issues.
Does anyone have any more info? I find super strange that is gets worse with the rear (only) windows down. It leads me to believe it could be a driveline or structure issue.
Any input is appreciated.
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