It would be interesting to see the internal correspondence between Nissan's marketing and engineering departments when the whole Cummins XD Cummins program was in its infancy. Did they originally intend the Nissan XD Cummins to be a "tweener" with 1400-1500 lbs. of payload? Or, did engineering (or their bean counters) promise something they couldn't deliver?
IMO, the Nissan's XD Cummins' apparent lack of a full-floating axle is its potential downfall. Certainly the frame and differential are in "tweener" territory. The Aisin tranny is HD capable. As for the suspension, increasing payload with a couple more leaf-springs is a cheap, low R&D fix.
This truck only needed a measly 400-500 lb. higher GVWR to make it really competitive and a real "tweener".