WD40 is useless in an application like this. Use some PB Blaster or Kroil on them. Start soaking them and I'd also advise that you got on a nice drive to heat the engine up, then pull the plug boots off and spray that stuff in there.
x2....drive the truck till the engine is warm, pull the boots off and soak with pb blaster, or Ive heard good things about wd-40 blu torch
soak the plugs, leave them sit for a hour before you try...if they are still stuck, re spray and try again in a hour
whatever you do, don't crack one off, and when you do get them out, a little grease on the threads will help you...
tip, pull your plugs one time a year to look at them and gap them. every time just add a little grease to the threads...you will never have this issue again...ever!