back pressure...please don't spread that nonsense anymore.
+1 more for changing out for better mufflers. just put in some dynomax ultraflows, hooker maxflows, hooker aerochambers, or something similar. all will give you what you are looking for when heavy on the pedal, and give you a nice mellow drone free highway sound.
i run a 20" case ultraflow on my pickup, with cats, and you can't tell a difference in the cab from the stock setup. it is, however, much louder and deeper on the outside.
in reality 2.5" off your manifolds to a y-pipe and 3" in/out muffler or a 2.5" dual inlet single 3" outlet muffler will give you the best performance all around.