http://www.cherrybomb.com/catback/15/83 Here is what Cherrybomb recommends for jeeps.
Im running the crappy Raptor (already was on th Jeep) and it sounds gay. So I was at Autozone and asked them for the quietest muffler they had in 2.5". They gave me a Cherrybomb Turbo. So i stuck it on behind the Raptor. Quieted it down a good bit.
To make a tail pipe, i just grabbed some 3" that i had laying around. I was in a hurry. Then I got around to actually doing it right. I stuck about 3ft of 2.5" pipe on over the axle and turned out.
Just changing the tailpipe dia made a difference in performance. I noticed that it now takes me just a tad longer distance, to accelerate up to any given speed. But I now seem to have a tad more power when accelerating up hills/inclines.