Many of the "trifecta" lights (ABS/4x4/Yellow Brake) threads on this and other forums relate to an issue that is specific to the later XDrive equipped X5 models (late 2003 and up) so you might want to concentrate on solutions for this issue on the older models with the simpler transfer case (no actuator motor).
On the older models the issue is often either a faulty ABS module or wheel speed sensors but once it was even an alternator!
