Whats wrong with my truck??

Okay, I have a 91 b2200 with the standrad 2.2. Nothing fancy. When I bought the truck, it had 7 leaves in the back due to it being used as a parts truck. After lowering it (the day after I bought it) I noticed the thing seemed to be lacking something. I replaced the rearend, but it still seems like it is in a lower gear than it is. What could it be?? ANY help would be GREATLY appreciated!!