Slow, sluggish, what should I change first

Hi. My truck has been slow and sluggish at around 40mph+. I can get up to 30 no problem, but once I hit about 35 its just seems to lose power. My gas milage is also terrible. It was 16, but now is up to 17.5. I read the Truck Not Running Right? Hard On Fuel And Sluggish Power? Try These First..... (http://www.automotiveforums.com/vbulletin/showthread.php?t=453376) but don't know what to do first. I've done some of that already. After I got it I replaced the air filter, changed the oil and oil filter, FlowTech Afterburner with dual outlet installed, had it tuned up w/ new plugs, [dk if Pep Boys changed wires], new serpentine belt tensioner [was about to go out], and changed the fuel filter. I don't know what I should change next. thanks

98 2.2 Auto Extended

i have a 93 4.3, and a few months back mine would onyl do 60 floored, and a lil while later the fuel pump went out, chnaged that and it has its power back