Ford 9" rear ends do not have a rear cover. 12 bolt and Dana 60 rear ends come standard with a chrome rear cover, or you can upgrade to an aluminum rear cover.
The aluminum rear cover is a performance cover that strengthens your rear. They are made from 356-T6 aluminum and designed to improve ring and pinion gear life, as well as improve the appearance of your vehicle. They greatly reduce the amount of ring gear deflection and therefore, your gears last longer.