Quino - This is my first BMW, rather happy with until this. When I had it at the dealer to check into this racket they informed me of the situation. Transfer case and front drive shaft replace = $4400.00. Even the tech that worked on it could not believe the cost. I found a used one through the net with 20,000 miles and swapped it out myself. Cost was less then half for the transfer case and front driveshaft. I am an experienced mechanic but I did not buy my X3 for this type of work. That is what my classic 65 Chevy is for. I dis-assembled the original to see exactly what the issue was. I figured it must be the chain, nothing else can stretch like that. The loud/hard knock under 1st & 2nd gear torque (manual gearbox) was the chain hitting the case. It became worse over the past few weeks, to a point where it was an issue. Early on was the whining/growling of the bearings. They did not seize but they would need to be replaced. Dealer could never find the issue. Fluid was replaced at about 40,000 miles and was not burnt when I tore into this thing. I am actually trying to get pricing on the spares from BMW to rebuild it. The dealer only offers the assembly. I am doing some checking with my colleagues in Germany now to see if they can track the needed spares for me.
Regarding sensors, the answer is no. Seems like the only electrical component is the actuator motor that engages the clutch pack to direct power to the front output shaft (for the front driveshaft).
Hunting a used transfer case is a little tricky too, have your VIN available. Even the front drive shaft was based on the monthly build cycle of the vehicle. Mine is a 2006 with a build cycle of 05/06, this means a lot when hunting used spares. Don't buy from someone who does not quiz you about Vin numbers and build cycles.