Okay. Update on this issue.

Replaced rear suspension this week.
RealOEM.com * BMW E53 X5 4.4i REAR AXLE SUPPORT/WHEEL SUSPENSION

This included both lower control arms, ball joints, integral links, upper control arms ( 06, 11) in the diagram.

Ordered aftermarket axle to replace right rear - didn't like it at all - housing was thicker, only 4 threads going into diff case instead of 8 threads like the bmw axle, and the flange on the differential side was different. Decided to order OEM axle and use that. OEM axle has not arrived.

Thusly - with all new rubber, the vibration is still present. No change.

Next week it gets the new axle (soon as it arrives). Then at the end of the month it gets new subframe bushings. I really think this is either axle or transfer case. We'll figure it out, but I think it's axle. It's coming from the rear, but only when under load - either acceleration or deceleration. I'll keep this thread updated...