I have a 1991 F150 300 Auto, After changing all the plugs the truck starts and runs good in Park. When I put it in Revers or drive the plugs start to foul out. I think it is the trans pulling down the engine to much. When I pull the plugs out they look like I have painted them with flat black paint, and they are dry.

 

Make sure you have the correct plug. Also, it this the original engine or a rebuilt?

I had a remanufactured(4.2) long block installed in my '97. A few thousand miles later I found a cracked plug. I was naturally now also 1000 miles from the dealer that did the work. Local dealer wanted $85(typical charge) to look at it. I changed the plugs myself. Not thinking, I installed plugs for '97. It sounded like a machine gun once it warmed up... not to mention the loss of power. Rechecked the plugs. The remanufactured engine was from '99. The plugs are different. Installing plugs for a '99 fixed the problem.

 

How are your wires? Putting the extra load on the engine in drive might be maxing out your old wires . If the wires are resistant and not leting enough energy through to the plugs it can foul them out. The missing might just be the wires and not the plugs at all.