This is your problem. You can do it yourself. BMW will say its your transfer case and say it needs replaced for thousands. The first link is the gear you'll need changed...and the second link is how to change it yourself.

BMW E53 x5 E83 x3 Transfer Case Actuator Motor Odometer Gear OE 27107566296gr | eBay

https://www.youtube.com/watch?v=bh6HvYcn-HA