Old thread, but since it seems to fit my situation, I'll jump in.
Working on the rig, crawling around underneath checking things out just to learn what I have, I noticed this sensor. I'm guessing it is what this thread is referring to, but I have steel suspension, not air. Any idea what it is used for on mine? I haven't checked to see if there is one in the front yet. This is the left rear. There is an empty bracket on the right rear.