Pinging, gas mileage and performance are all gone now. Try the reprogramming solution first. Do it with a new tank of regular gas, so it learns with the gas you use, in conditions you typically drive in. Maybe a half-hour of driving will do it for you. Use the shorting-block trick only if you can afford the poor mileage.
New plugs are cheap.
Oil or coolant in the chamber will cause pinging. I had a pinging problem in my Explorer but fixed it easily. If it's off, it could cause a lean condition, causing a ping. I've seen about 10 of them do that. Here are some nice photos and the procedure for a 1994 Explorer.