The rear drums tell them what brake package is on your truck, that is assuming that the rear end has never been changed. If you have ever tried to interchange calipers, or rotors, or brake hoses in front there are two kinds that will not work with each other... I learned that lessen trying to take a caliper from a 86 Suburban and try to put it on a 86 GMC, nothing worked even though the pads were the same...