I drive an 03 Lightning.

I recommend checking the exhaust headers for any cracked bolts (assuming this is still stock.)
Plugs are scheduled for a swapn out after 100K. As if they have been done or not. If not Ford dealers charge between $450 to $500 to change out plugs, boots and coil overs.
BTW 2nd Gen L's from 99 to 2002 had only 3 threads holding the plugs in. Many problems with th eplugs blowing out of the head and THAT's a pretty penny to fix. This was FIXED in 03 when Ford made plug threads with 6 turns instead of three.
So unless it's an 03 or 04 then pull the plugs to see if any are different than the others and ask the question if they have been changed out or if one did in fact blow out.

I will NEVER get rid of my 03 Lightning. One Bad *** Truck,

Enjoy,
King Leo