I had the same problem. I just took the hitch receiver and cut/knocked out the back plate. The back plate does nothing other than keep you from inserting something to far and hitting the bumper. As long as you're careful when you insert something you should be fine. If you have a dremel, a punch and a hammer it takes about 5 minutes to remove the plate.
I would rather remove something that is not needed from the receiver than to void the warranty on an expensive bike rack.
I asked the same question at
http://x3.xbimmers.com/forums/showthread.php?t=684224