I wonder if it has anything to do with the crossover inside the muffler right before the exit. How far out do the pipes come after the muffler before the turn down? Wonder if it turns down too close to the muffler. My wife says during really cold weather that my exhaust will rattle stuff on the walls of the house for an instant when I start it. It's definitely not that way in the cab.
Looks almost identical to yours. I would say about 6" after the weld they turn down about 45 degrees. You can not only hear the drone you can feel it. Resonates against the body so much you can feel it in the seat.
I have a feeling the low end frequencies are bouncing back & forth between the pavement & the bottom of the truck.
As I said before I used to be an audio engineer & it's mind boggling how sound waves work. Maybe yours is only 5 degrees one way or the other & that's why yours doesn't do the same.
I think the easiest remedy would be just to run them all the way to the back, paint em' up real good & hope they last longer than 2 or 3 years. Heck with the way winters have been the last couple of years I won't need to worry about salt!
I think running it out the end will keep it from pushing up through the floor board. There goes more $$$!
Hope momma doesn't find out!