Not that it is a huge problem, but one of the weird things about the generation one expeditions was they could shoot the spark plugs out of the engine unexpectedly. It happened so commonly in the motors used in expeditions and Crown Victoria's, etc. that a TSB came out for the problem. You would think that losing a five dollar spark plug is a five dollar problem. But if it rips up the threads on the way out and it is one of the plugs that is towards the rear of the engine it can be a very difficult and expensive repair . In the worst possible scenario, removing the head from the engine might be necessary to Machine the spark plug hole and replace the spark plug. Thats like a 15 hour job!
As was mentioned earlier if you can find a low mileage 99 through 03, it can be a great truck, but make sure you check the torque on all your plugs before you set off down the road.