The PCM in your truck has a misfire monitor that recognizes misfires. Misfires can be caused by lots of things. An injector would however probably be at the bottom of my list. You can check it along with the plug and plugwire from that cylinder realitivly easily though. You can move them one at a time to a different cylinder and see if the misfire follows. My experience with Mag engines of that era tend to have clyinder head issues though. I'd do a compression test to see if all of the cylinders are equal to make sure there is not a valve sealing problem. Also pay close attention to the spark plug out of the #1 cylinder compared to the rest of them. Look for traces of coolant on the plug and overall conditon. The mag motors are prone to cylinder head cracking, especially on the end cylinders.