it could be alot of things, some things to ask:
what initiates the misfire? light or heavy load?
does it do it when the truck is warmed up or cool?
how many miles are on the motor?
how old is the cat, it could be clogged?
if it were my truck i'd start with checking the usual, fuel pressure, compression, signal to the injectors, etc... it could be a faulty coil. check to see if your getting a spark, and that ALL wires are tight and have a clean connection points. i cant tell you how many times i have seen hundreds of dollars spent due to corrosion or a loose wire. injectors arent a bad idea, just an expensive one. or at least more money than im sure you wanna spend. take the truck out and play around, see what pisses her off. try to duplicate it at a lower rpm. i've got a 99 3.slow, their tricky lil motors when they get problems like this, but almost every problem i've seen on mine has been nickle and dime fixes. keep me posted man!
99 Ford Ranger 5-Speed manual 3.0 Flex Fuel "work rig"
07 Jeep Grand Cherokee 5.7 Hemi Limited "wife's car"