Turn the truck off and then make sure you can easily take the sparkplug wires off. Then start the engine, get a leather glove, and while the engine is running, pull the sprakplug wires off one at a time. If you pull a wire and it makes it worse or stalls, then you know that one was working. Start it up and go to the next one. If you get to one that does not make any difference if it's plugged in or not, that is the dead cylinder.
If you find a dead cylinder, stop the engine, pull that wire off and stick a screwdriver up in the wire. Start the engine and hold the screwdriver by the plastic handle, and put the metal part of the screwdriver very near something metal on the engine with it still stuck up in the sparkplug wire. You should see a spark jump from the screwdriver to the engine.
If you get spark, the next step is to get a compression checker. If you do not get spark, you need to troubleshoot that wire or coil.
