Reviving an old thread, but something else occurred to me the other day while messing with my Jeep... I have a Gibson exhaust because I thought most of the top brand exhausts sounded like crap on a 4.0L, and the Gibson was touted to be quiet. It is fairly quiet, but DEFINITELY louder if I put on the large stainless tip that came with the system. Somehow, that tip makes it more boomy, and causes it to actually have an annoying resonance at about 1800rpm that isn't there without the tip.
If you have a system with the large tip, try removing it!! Even if you have to add a short section of tube clamped on to get the exhaust to exit in the right place, it might make a noticeable difference in your ride comfort!!
Just something free to try.
Chris