There were a few years where the 3v modulars had issues with spark plugs seizing in the heads. There have even been cases of spark plugs blowing out. It's not an uncommon issue.
Here is a TSB addressing the issue. It addresses removal of the broken plugs. When you get the plug out and install new ones you absolutely MUST use antiseize on the new plugs.
A friend of mine has a 2005 F-150 with over 110,000 miles that needs plugs. I know I'm going to end up doing it. Not looking forward to it.