ABS Sensor. It's practically useless in a Jeep and I haven't heard anyone of wiring it in to make it work. Just unbolt everything and pull all the mechanics out. You might need to remove the differential to remove some of the stuff from the inside I don't remember. All I did was take a piece of scrap metal, cut a hole in it, put gasket sealer around the outside the bolted it in place.
You'll also need to grind the fins down to allow the track bar to fit. I just used an angle grinder and flattened the fins completely down.
Ironrock also sells a bolt-on piece, but if you have gotten this far, making one yourself and using the bolt already there is a piece of cake.