The official notice
Defect
On certain 328d xDrive models, engine vibrations could damage the rear driveshaft joint. Over time, this could cause the driveshaft to separate, which would result in a loss of power to the rear wheels. If the driveshaft separates, the vehicle could also move when the transmission is in Park, and the parking brake is not used.
Safety Risk
A loss of power to the rear wheels could increase the risk of a crash. A parked vehicle that rolls away could also increase the risk of an injury or a crash.
Remedy
BMW will notify owners by mail and instruct you to take your vehicle to a dealer to replace the rear driveshaft constant-velocity joint. Dealers will also inspect the transfer case, and if necessary, replace it. Note: BMW recommends that you should use the parking brake, at all times, when you park the vehicle. This will minimize the risk of the vehicle rolling away.