Pauls14,
If you have verified the "alleged" tune up to be actual, here's what I would do:
1) complete the tune up with new engine oil, transmission and axle fluids, engine coolant flush/fill and verify your tire pressure.
If mileage is still crap,
2) have your valves checked and subsequent valve job performed by a competent, trusted machinist.
If mileage is still crap,
3) look at your valve train components. The rockers and push rod wear are known issues on these power plants. Also, you may want to consider the use of adjustable push rods so that you can maintain proper valve geometry in the future.
*Deltacam of Tacoma Wa is the only company that refurbished worn rockers and adds a hardened insert.
* Smith Brother's will make you a custom set of (adjustable or non-adjustable) push rods for your engine, ensuring the highest possible efficiency from your lifters and valves.
There are more moves to increase your 4.0L's fuel economy but that's were I'd start.