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.