The official notice
Defect
On certain vehicles, the connection between the driveshaft and rear differential could wear. If this happens, there could be a loss of power to the wheels, or the vehicle could move when it's parked and the parking brake isn't used.
Safety Risk
A loss of power to the wheels could increase the risk of a crash. A parked vehicle that rolls away could also create the risk of an injury or a crash.
Remedy
To reduce the safety risk, BMW recommends that you should always use the parking brake when you park. BMW will notify owners by mail and advise you to take your vehicle to a dealership to inspect the connection between the driveshaft and rear differential. The connection will be repaired or, if necessary, the driveshaft and rear differential will be replaced.