Just changed my tfr case fluid today on xDrive ATC 500 unit - simple and quick. Dealer to reset actuator value tomorrow.

This is a also a good time to move the condensation water outlet hose so that it does NOT drip water on the drive shaft just before it enters the transfer box (mentioned in another E53 thread). Loosen the rubber mounting bracket with a 10mm socket, adjust the hose so that it drains directly to the ground and tighten up again. (May wish to do this next time you drain the engine oil?)