I feel your pain - took me 1000s of miles to get my 2013 to ride smooth
see: http://www.gm-trucks.com/forums/topic/150488-new-2013-vibrates-at-40mph-and-dealer-claims-normal/page-5
I realize this is the new truck, and could be many things, but after my expereince I would want to be very comfortable with the tires being correctly balanced before I let the dealer start tearing things apart.
What brand of tires came on the truck? My goodyears were swapped to bridgestones (after trying about 3 sets of Goodyears) and my issues went away like magic...
My vibration was very similar to what you describe, would come and go depending on road surface, and even if they got it right, would always reappear at about 2k miles from last balance with the goodyears. Do you know what the Road Force numbers are for your wheel/tire combos right now? On the GMT-900 trucks spec is around 24lbs and mine had to be in the <12 range for the issue to disappear. I have about 6k on the bridgestones now and they have been great/finally allowing me to love my truck.