stock.

really?How big of a problem is it,and what causes it?

If it's the rearmost plug on the right side (#4), it's usually due to the plug being cross threaded during a plug change. A lot of the cause has to do with the access and angle the plug is in the head - they face back on the right, forward on the left. They are tough to get in right, and if a tech puts a wrench to it without screwing it in by hand first, it's WAY too easy to cross thread. If it IS that one, you may not be able to do it from the outside, simply due to the inability to get to it, and you'll be forced to take the head off - or remove the cab/body to access it!

Its #3

You may be lucky - they may have enough access to it! Just curious - how long since the plugs were last changed?

less than 30k miles

From the Ford Truck message boards, the earlier V-10's are known to blow the spark plugs out too. I think there is only 3 threads or so that hold them in. 3 threads, aluminum heads... don't need to be a rocket scientist to figure this one out.
Proper torqueing is a must. The timesert seems to be the most prefered repair, otherwise a new head was ordered.

I have blown out two on my old lightning and that was with 18lbs of boost and a 100 shot of gas, heli-coiled both of them and never had a problem with them........some poor sucker might up the road, but it ran awesome for a year and a half under heavy abuse and held just fine.

Rerminds me of my days in college! I used to work on mostly friends cars for extra cash. Onna my friends came to me after already spending a few hundred telling me he needed to spend somin like 6 or 7 hundred on his old buick because it needed a timing chain! I told him I would look at it. The *******s had run the timing up. 5 minutes and he was on his way with a car running like a champ. That 200 and somin $ tune up almost ended up being a grand! If he had come to me in the 1st place he would have left spending less than a 100! :D

PS Got a heck of a job with the local newspaper working on their fleet for the same reason! tried and tried to tell them as I was driving for them...when a motor finally blew I had been warning them about the offered me a job!~ You wouldn't believe the sh*t I saw! When I first took the job onna the trucks had just had a brake job. The back brakes were toast in a couple a days. They had run the emefgency brake cable tight! Got me a free pair a vicegrips outta that one! They were still locked on the adjustment sleeve! :D

a heli coil insert is light years stronger than the original thread