There has been some issues with the 4.6 and 5.4's blowing out sparks plugs. Blowing them right out of the head and taking the threads with them. Sometimes, major engine damage has resulted. Almost ALWAYS a ticking like an "exhaust leak" was heard shortly before the plug came out. This has happened to literally hundreds of trucks from the showroom floor to 100k+ miles. Ford knows of this but has done nothing about it. The lowest mileage I have ever heard about this happening with is 2k. Being at 2500, you might want to discuss this with the tech.
There is a thread here regarding this "known" problem.
Blown Plugs, damaged cylinder head
We have 10 give or take F150's at the AFB, 3 have had the plug puking syndrome. All govt. owned so you KNOW service has been on the dot.