Start the engine and let it idle.Pull off the spark plug wires one at a time(wear leather gloves or use a plug wire tool to avoid being shocked).The wire that you pull off that makes no difference to the way the truck runs is the problem.If a set of plugs and wires don,t fix the problem,remove the suspect plug and check to see if it is firing by laying the metal portion against a good ground and then have someone turn over the engine while you watch for spark,if you have spark then you likely have a plugged catalytic converter,if there is no spark then you need a coil pack.
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