I have the same problem on my 2000 BMW x5 4.4i.
I was driving it and then self-leveling message came on the dash.. day after day the rear suspension started to get higher. a couple of days ago, i sit in my car and felt that something was weird. I went to see the car from the back and it was sitting on the ground. Suspension dropped and it was one bouncy ride. Do you guys think its the same problem? If i get the sensor, will it fix my problem?