Your right it ended up being a resonator. Looked like a cat till I started looking more closely. That poor old pickup finally gave out. It did good with the new lift pump till I pulled my trailer with 5 cows. Did great till I got to a hill then lost all power and died. Had to get a tow, mechanic said total rebuild to get the poor old thing going again. Ended up trading it for a 08 Cummins. It was a sad day. But I have to say that 6.7 is insane. Hope they're as reliable as that old 5.9 was.