mhughes and hahns are right.
most of the noise your experiencing now is because of resonance due to the exhaust exiting right under your feet. it you pipe the exhaust all the way out the back of the truck, the decibel level inside the cab will be quite reasonable.
when i first got my truck, it had the stock exhaust, and the muffler had rusted completely in half, so it was in effect the same as just taking the muffler off. back then, the sound level in the cab was much higher than it is now with my true duals and tiny glasspacks...even though the duals are much louder when your outside the truck.
if nothing else...it'll only cost you a $5 peice of patch pipe to give it a shot