Do you have a service manual? If you check it out in the IG section ignition troubleshooting section it says:
Poor Fuel Economy:
Spark Plugs faulty or Incorrect iginition timing. Also High Tension Cords disconnected or broken (plug wires). Pull them off one at a time and put it on the fender and have some1 crank the motor for 1-2 seconds w/o starting the car. It should spark if no spark replace cable.
Good luck! It could be something stupid like a dirty air filter. Maybe you need an oil change maybe one of your cylinders isn't firing. Fuel filter wouldn't cause bad mpg but it would make it hesitate. An exhaust leak could also cause bad mpg because of loss of back pressure and you would have to rev the engine higher to get to the same speed limit as a car without a exhaust leak. Process of elimination. Do you maintain the 4wd with grease? I know it isn't engaged but it is free spinning and when it isn't cared for properly it will create some form of drag.