I went thru that horror with my 06 Explorer with 4.6 3 valve motor.
At 99k miles # 8 stopped firing so I had no choice. I soaked them, used torque wrench and still managed to break that # 8 but in much worse way. It snapped right below the hex part leaving the tread and electrode in the head. I used Leslie to get the electrode out and then drilled the treads, and tapped in a bushing (Dorman 42025 from a 2 valve Triton), and reinstalled a new plug. It's not a perfect solution, but I am at 180k miles and it works just fine.
What I do now is every summer I loosen them all, in my case that's every 23k miles. Even that often, they will still give you a good resistance and higher blood pressure :)
