Definately plugs. You have the 5.4 3V engine, don't you. Be advised that some of the plugs will most likely break, nesecitating for you get the Lisle tool to pull them out.
If you are taking it somewhere, ask what their price is for the change, how much per broken plug, and how successful they have been.
If you haven't replaced your fuel filter, it's probably time for that as well, amoung other filters.
Just follow the link above...