I just did this today. I realize your question is 10 years old, but your old question is someone's new one. I have a 2012 with a 2.4 that had a rusted and holey single muffler that sounded awful. I ordered the 6 cylinder dual exhaust muffler with great looking tips from Rockauto (Walker 50081) for just a bit more than the 4 cylinder single tip. Rubber mounting points appear to be the same, not sure if exact, but all 3 are hanging just fine. There are 2 issues. 1)Cutting out holes on the bumper as these duals sit higher than the single. Just hang the muffler with the tips under the bumper and cut up after you're sure it is centered right. I used a grinder. I'm going to hot knife cut around the tips better in the next few days and perhaps get some kind of edging to make it look better than just a cutout. I'm not buying another bumper. 2) The intake pipe on the 3.0 muffler is shorter and points down like 25°, it doesn't have the necessary bend that the 2.4 muffler has to meet the middle pipe. I found an almost perfect solution at Autozone for $8.99. It is a 2 1/2 inch exhaust tip with an almost perfect angle. I cut a little here and there and about 2 inches from the center pipe on the truck and oiled up the connections and straightened it out before clamping. I stretched out the intake of the muffler with a 2 1/2 inch rent-a-tool also from autozone because it wasn't perfectly round. I overstretched a bit, though. It sounds quiet, no rumbling at all, and looks fantastic.
