Ok sitting here wondering if I bought the wrong car... However, doing some math comparing the suburban, which backs up just fine, and the X5, it appears that the X5 should actually have higher torque at the wheels in reverse than the 454 suburban.

Assumptions:
R X Final X Torque = Wheel Torque (is this right?)

Suburban
R:2.085
Final: 3.73
Total Ratio: 7.78
Torque: 385
Wheel Torque: 2994

BMW
R: 3.295
Final:3.154
Total Ratio:10.39243
Torque:300
Wheel Torque: 3117

Is that math right?

This ZF box will not handle uphill tow in reverse - you are lucky it was not damaged.

Well that was a minor detail I overlooked. Thanks all for your input.

Elaborate, where are you getting this info?

+1.

More info from my digging... Talked to a ZF-certified transmission shop owner, and he thought I was describing a properly operating transmission. He's never heard of any issues with reverse like I was describing on the ZF8HP. Since the RPMs were staying low, that is an indicator that the transmission wasn't slipping.

I also removed "X5" from my google searches on backing uphill with trailers, and it is not an uncommon problem with vehicles that do not have a 4wd-low gear range.

Now trying to figure out how to fit that circular driveway in our front yard.

On this 8HP45 car - in reverse if the load is high as determined by the DME - engine torque is reduced (engine rpms reduced as you experienced) to protect the transmission...had to look up my notes about this complaint from another car.

Is this the same for the 6 speed tranny?

I dont think so. I back-up my race car and trailer up to a short, relatively steep driveway and never felt power reduction. Perhaps there was not much room for the reverse gears and the 8 speed and they made them really tiny and fragile.

Thats my experience as well, I have backed up my boat with no noticeable loss of power.