I bought a 99 E-150 with a 4.6L V8. I went to change plugs & wires and the #2 plug wouldn't take a socket. After buying an inspection scope so I could see the plug I discovered that the last person to change the plugs had left the rubber insert from a plug socket on the plug then forced the plug wire on and shoved it down to the bottom of the plug. I tried a lot of different things fro 2 1/2 days then picked up a rubber grommet I had sitting on the bench that was similar rubber to the insert. After looking at it for a while I wondered what Acetone would do to it so put it in a tuna can covered with acetone for about 1/2 hour and checked it. it was starting to fall apart so I used a flex neck ATF funnel to put a couple tablespoons af Acetone down the plug hole. Let it sit for about an hour the wicked the Acetone out with a paper towel and blew the rest out with an air hose. The insert was still there but softer so I did it again and let it soak another hour. When I removed the Acetone and blew it out again the insert was completely gone! A plug socket fit on normally and was able to remove the plug and install the new one.
Hope this saves someone a lot of frustration.