I was getting much better mileage than that when I was stock and I have the 4L60E 4-speed tranny (assume you have the new 6-speed). I also have the 4.10 diff gearing and it has been discussed that this keeps helps keep the truck in v4 mode more often/longer. From memory I was getting ~16 mpg city and ~19 mpg highway at 70 mph.
Are you going by the DIC or hand calculating mileage? Which final drive ratio do you have? Did you change out the transfer case and diff fluids yet?
IMO Black Bear tune will help out. See their website and search this and other forums. I had mine tuned after the level kit and larger mud tires, and gained the mpgs. Some of that I attribute to speedometer correction for the larger tires. After the tune it is hard to keep off the skinny pedal though!