With them exiting under the bed, you have turned the bed into a sounding board. The exhaust needs to exit past the rear axle out the side, or past the bumper to not resonate under the bed.
As for making a really quiet system, tie the two sides together into a 3" resonator, then use a 3" in, and dual 2.5" outlet muffler if you want dual exhaust tips, or a 3" in / 3" out if you want a single exhaust system.
