Is the truck new to you, or did the problem just develop? Sat for a long time? I’m not familiar with this engine, but if it’s an overhead valve. Pull the valve cover, check for obvious differences in the valve train on that cylinder vs the others. If nothing is obvious other than that exhaust rocker has more slop than the others and it has an umbrella seal on the valve stem, slide it up and spray your favorite lube, Kroil, PB blaster, ect under it. Then LIGHTLY rap on the valve stem with a brass or dead blow hammer, spraying repeatedly, using caution to not hit the spring retainer. This will usually free up a sticking valve from not running for a long time. You will notice a change in sound pitch from the valve when it frees up and starts contacting the seat.