I just had the same issue with my 2011 Titan with 58K miles. Just makes me think the exhaust design and material is terrible. The flange over the axles was so rotten that I pulled it apart with my hands. Also the back of the muffler had all sorts of rot on it. I replaced it was a stainless Gibson Swept Side exhaust that I bought ton Amazon. It was $440 shipped to my house so I was happy with the price.
Overall the install was pretty terrible and not because of the Gibson exhaust. The P.O.S. stock Nissan exhaust was rusted and the studs on the back of the cats would strip, break, or bend. The first bolt I stripped the nut and used a removal socket that ended up breaking the stud. The second stud, I heated up cherry red then hit it with the impact, got the nut off but destroyed the stud in the process. The short version of the story, I ended up cutting all the studs flush and drilling them out. Then I used stainless bolts, nuts, and lock washers to assemble it again.
Assembling the new Gibson exhaust took me an hour but removing the old exhaust and fixing the studs on the cats took me at least 4 hrs. I was not a happy guy. I think the quality of the hardware the Nissan uses is total garbage. I have never had as many stripped nuts as on this truck.
The Gibson exhaust is louder than stock but not obnoxious. My only complaint is that is drones a little at 1500-1600 rpm which is around 40-45mph with the Big Tow package. On the highway is sounds stock which is great. 70-80 mph is is dead quiet. Gets louder around 60 mph. I don't notice any gains in performance or mpg.
