Carbon forms on the rings and in the ring lands (grooves in the piston) and prevent the rings from floating freely in the lands...this keeps the rings from controlling oil and blow-by. I'm surprised someone hasn't used a leak down test to diagnose this?? To decarbonize, they add a solvent that soaks into, and dissolves the carbon, thereby allowing the rings to move freely again...driving it hard won't do this. There is a homebrew to do similar in small engines, not that I recommend doing this to a $$$$$$ truck under warranty; but I've used Kroil before to free up stuck rings in Gravely and small Kohler engines. I can't tell by their description if its a fuel issue or oil issue that causes the carbon?? Carbon deposits in the tailpipe are normal under all driving conditions...its a byproduct of the combustion process (incomplete combustion), which happens even at WOT.