We had same problem and after reading this thread I checked and fix mine this morning. Thanks to all.

 

Found the same loose nut on mine at 41k miles. As soon as I was finally able to reach the nut, it slid right up the piping so it was not attached at all and I have noticed exhaust smell in the cab about the last 5k - 10k miles at times.

Thanks for the head's up on this easy, although difficult to reach, fix. Find the longest 5/8's wrench you can!