I was in nearly the identical situation and it turned out to be an injector. A quick and dirty way to check is to disengage your distributor at the coil (eliminate spark this is very important), and then remove your fuel rail and work it so that the injectors stay in the rail. to do that, I got all of the wiring harnesses out of the way and took out the 4 bolts holding the fuel rail and jimmied it till it came loose (this may prove to be difficult, they can be pretty stuck in there). Once you have it out with all injectors intact and the dist. disconnected, have a buddy turn over the engine a few seconds at a time and watch the quality of the atomization (misting) for each injector (should put a dark towel down to spray into). They should all spray the same strong mist. Some might not like this way of testing but it's free/no tools or parts required. Other than that towel. Just remember to disconnect the coil so you down burn your truck down.
ALSO: a quicker way to narrow it down would be to simply pull the wire off each plug at idle to see if you have a dead cylinder (when you pull the wire off, the engine should struggle. If pulling the wire off doesn't change a whole lot in the running condition at idle, you have a dead cylinder and I'd bet a paycheck on it being the injector for that cylinder. I say this assuming you did a sound compression test and fuel pressure test and changed all those "tune up" parts. NOTE: wear a glove or make sure to keep your fingers away from the end of the rubber boot on the wire. You may have to find out the hard way what happens!
cheers
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