Forgive me if this has been covered already, but what year is your truck? What engine? You may want to go to USERCP>Edit Signature and add your vehicle specifics to your signature so that they are out there as you move through the boards with different questions.
Tuning your truck for mileage can be a good idea, but which "tuner" to buy depends a lot on what all you want to be able to do with it. Would you like something that is just plug and play with a little more power/economy and that's it; or would you like to also be able to check and clear diagnostic trouble codes or monitor engine operating functions like timing, etc.
A K&N intake can help, but by itself it may not have as much effect as you might like. Improving air flow into the engine is good, but it would help to increase air flow out of the engine as well with a better flowing aftermarket exhaust. Together with a good tuning option could have a decent increase in mileage.
Other things to consider are, if you haven't already, install full synthetic fluids throughout your drivetrain and tire pressure. Running synthetics will help with mileage as a result of their uniformed molecular structure that provides better flow characteristics. With the tires, it is a trade off, but running the max PSI at all times will provide the best possible mileage, the trade off is increased premature wear on the center portion of the tread, leaving you buying tires more often.
j/k i do feel ya though, gas prices are crazy even though i live in a area with the lowest prices in the country... still not cheap though, thats why i love my neon and matrix, the matrix gets over 40mpg highway, but if i could afford it I would drive the tahoe everywhere, such a comfy ride.
