Hbarlow, eagerly awaiting your findings from cummins. I pray there is something holding this truck back otherwise it is the least powerful cummins I have owned by a good margin (I have owned 4, both stick and auto, 2 hpcr). The power is also lacking where it is needed most, bellow 2000rpm. I pulled a 4500lb 23 pro line yesterday which should be a joke but had to wind the guts out of it to keep pulling in the next gear (up hill). I am a cummins fan but this is silly!
I'd say your trouble is with gearing.
Even with the g56, 3:42 gears and 35's are going to zap a good bit of torque...