Just went through an exhaust overall myself. My cat was shot. It rattled INCREDIBLY loud and my power was down as well as my mpg. I was getting maybe 7-8 mpg on the highway.
I replaced my cat with a magnflow direct fit from O'Reily's for $150. After taking off the old cat and looking into it, it was broken into two to three big honeycomb chunks of ceramic that was loose inside. Tipping the old cat would result in small flakes of ceramic flowing out of it. We decided that the muffler was most likely plugged up with ceramic flakes and such, so I put in a Flowmaster Super 44.
When it was all said and done, the repair and upgrade makes the Jeep run MUCH MUCH better. It's like night and day. And it sounds like a champ.