You may already have the parts you need if you have a good place to do the work. The most popular way to drop the back of a truck is to put the leaf springs under the axle. In a stock setup the leaf spring comes down and the axle attaches with a kind of U-bolt. All you need to do is remove the leaf spring, run it under the axle and re bolt upside down. It will drop about 2 1/2 to 3 inches depending on the diameter of your axle. Be careful because you'll be crawling around under the truck that is jacked in the air and you'll be dropping a pretty heavy rear axle. This will mean that you can't haul as heavy a load since you will bottom out the suspension easier which could lead to broken axles.
