The rubber thingy is the new pinion snubber. You must remove the stock one out of the car and replace it with the one provided. If you are unsure of it's location, if you look at where the driveshaft meets the rear end housing, it's directly above that. It's purpose is to stop the travel of the rear end, from contacting the bottom of the car. Since the Eibach's are 2 inches shorter, you must replace that.