what kind of maintenance have you done lately. the reason i ask is, if it has been a while, that would be a good place to find missing power...
plugs, wires, timing belt, air filter (k&n), and possibly change to synthetic fluids, engine, trans, rear. as for bang for the buck, i would think this would be the best place to start... depending on how you use your truck a rear end gear swap can feel like a 25 hp upgrade.