I don't understand why people think the 4.0 gets that bad of mileage. I have pulled numbers with it like 21 before on the highway, and mine has 100K on it (180K on the rest of the truck). I've recently gotten about 15 because I have been towing and using four wheel drive (when it worked) a lot more. If you aren't a lead foot, and you aren't towing anything, the 4.0 could get about 18 on the highway, for sure.
I say keep the black.