You may also want to check the wheel bearings, but with the millage you are talking about, it would be more than just bearings. If you have the owners manual for the truck, you should be able to look at the numbers on the sticker located on the drivers side door and then look that up in your manual to find out what the differential gears are (or were originally). Originally, I was getting right around 16 with my truck. New wheel bearings, swap from 4.10 to 3.55 gears (and new seals and bearings), alignment, and Auto RX treatments and I'm now around 18.5 mpg. I removed the soup bowl 2 days ago, so we'll see what that does to my millage. Next thing will be to go back to stock tires. I'm hoping it will push me up over 20.
With what you are describing on your truck, I would probably start with what everyone else has already stated, injectors and timing.
Just my 2 bits,
Mark