Don't do what I did.
 

Went over to visit a friend yesterday. He has a long, steep, narrow driveway.
I pulled in nose first and went inside to help on a fireplace project.
After a few hours it had cooled down outside. I started the engine and thought
I pushed the button for the seat heater. I started backing down the long driveway.
The front end was popping, the brakes were chattering, the brake pedal was pulsating.
WTF???? all I need is a sick BMW New Year's Eve.

I backed down the driveway till the roadway. I then put the X5 into Drive.
I looked down and noticed that I had actually depressed the hill decent button.
Upon finding my mistake, I clicked off that button.

I'm assuming the hill decent button doesn't work correctly when backing down
a hill.
Easy mistake to hit the wrong (yet very close) button then wonder what's going on.

Did you try going back up the hill and descending again without the button pushed?
I've recently had a similar issue that is probably due to the front drive shaft splines going bad, not the hill descent button.
I'll find out this weekend when the remanufactured with 1" longer splines drive shaft shows up and gets installed.