Like the other Dakota Quad Cab owner, I get (and have gotten) around 13 mpg in the city and around 19 mpg on the highway with both of my Quad Cabs.
My '00 Quad Cab had full-time 4WD with a 'lock' position on the xfer case for 4-HI. My '04 Quad Cab has part-time 4WD (conventional), with an electronic xfer case and a dash-mounted switch.
I find the new truck gets about 1 MPG better than the old one in the city and on the highway. It works equally well in the snow when I put it in 4-HI.
So... if you want the best mileage you can with a pickup truck, get 2WD (or conventional 4WD) instead of full-time 4WD, and get a big enough engine so that it's not having to struggle, but not so big that all of those horses just sit under the hood eating whether they're being used or not, and forget about the little supercharged engines (they're really a bigger engine that eats whether you need those horses or not). And don't expect to get more than 20 MPG on the highway or 14 MPG around town.
If you're willing to live with a smaller pickup and all the compromises that entails, look at the new GM S-series pickups with the 5-cylinder engines. They run well, they're great for mostly-empty driving with the occasional load for utility, and they're relatively inexpensive... although the Dodge Dakota is bigger, offers more utility, gets almost as good mileage, and costs less.
