you can get your gas milage up considerably by simply replacing that ****** air filter with a high flow filter like a drop in K&N... under $80
Then add the Magnaflo muffler, its a straght through designe and sounds GREAT! should be able to get the part and have it welded up for around $150 if you do the leg work and shopping yourself. (got mine 100% done for $135)
i would also recomend getting the e-fan kit done, this dissconects the clutch driven fan and frees up some power.... I have seen this mod done for around $120 but thats you doing some serious deal hunting and labor yourself... otherwise you are looking at around $300
after all that is done GET A CUSTOM TUNE!!! this is the KEY element here... if you get those top three done up there and get a custom 91 octain tune, you can see a solid 3-4mpg increase.... when i bought my truck i was struggling to get over 14mpgs... i now stay stead around 17.5 and if i baby the shit out of it, i get between 19-20.... (i actually dont have the e-fans yet, but i want them and expect the gas milage to go up at least .5 to 1 mpg)
i also replaced my spark pluggs, fuel filter, plug wires and got a gold top battery... those things probably didnt hurt the truck either