Spark plug change

There is an individual coil for each cylinder and the spark plugs are located about 5 inches down inside the cylinder heads under each coil. And let's not forget that you need a 7mm 1/4" drive universal socket that costs 50 bucks to get to the 7 and 8 coils, and triple jointed fingers.
It is not an easy job to change plugs on these engines, we usually charge 3 hours labor to change them, it is not a job that the usual vehicle owner can do. I would also suggest you let the dealer change them, they have done many and will save you a lot of pain and time. I always end up bleeding from somewhere after changing plugs on this engine and the plugs are know to break.
Good luck and hope this helps. The plugs are rated up to 100,000 miles and most of the time it's the spark plug boots that go bad and if your getting a miss fire it's a good chance that it may be a Coil to one of the plugs. There should be a DTC code that would indicate if you have any issues with your engine performance.

When you're removing spark plugs, work on one at a time. Don't start by removing the plug wires all at once, because, unless you number them, they may become mixed up. Take a minute before you begin and number the wires with tape.

1. Disconnect the negative battery cable, and if the vehicle has been run recently, allow the engine to thoroughly cool.

2. Except on the 5.4L and 6.8L engines, carefully twist the spark plug wire boot to loosen it, then pull upward and remove the boot from the spark plug. Be sure to pull on the boot and not on the wire, otherwise the connector located inside the boot may become separated.

3. On the 5.4L and 6.8L engines, unplug the electrical connector on the individual cylinder ignition coil and remove the retaining screw. Remove the coil from the cylinder head, taking care in removing it from the spark plug.

4. Using compressed air, blow any water or debris from the spark plug well to assure that no harmful contaminants are allowed to enter the combustion chamber when the spark plug is removed. If compressed air is not available, use a rag or a brush to clean the area.



Remove the spark plugs when the engine is cold, if possible, to prevent damage to the threads. If removal of the plugs is difficult, apply a few drops of penetrating oil or silicone spray to the area around the base of the plug, and allow it a few minutes to work.

5. Using a spark plug socket that is equipped with a rubber insert to properly hold the plug, turn the spark plug counterclockwise to loosen and remove the spark plug from the bore.




WARNING
Be sure not to use a flexible extension on the socket. Use of a flexible extension may allow a shear force to be applied to the plug. A shear force could break the plug off in the cylinder head, leading to costly and frustrating repairs.

To install:

6. Inspect the spark plug boot for tears or damage. If a damaged boot is found, the spark plug wire must be replaced.

7. Using a wire feeler gauge, check and adjust the spark plug gap. When using a gauge, the proper size should pass between the electrodes with a slight drag. The next larger size should not be able to pass while the next smaller size should pass freely.

8. Carefully thread the plug into the bore by hand. If resistance is felt before the plug is almost completely threaded, back the plug out and begin threading again. In small, hard to reach areas, an old spark plug wire and boot could be used as a threading tool. The boot will hold the plug while you twist the end of the wire and the wire is supple enough to twist before it would allow the plug to crossthread.




WARNING
Do not use the spark plug socket to thread the plugs. Always carefully thread the plug by hand or using an old plug wire to prevent the possibility of crossthreading and damaging the cylinder head bore.

9. Carefully tighten the spark plug. If the plug you are installing is equipped with a crush washer, seat the plug, then tighten about 1 / 4 turn to crush the washer. If you are installing a tapered seat plug, tighten the plug to specifications provided by the vehicle or plug manufacturer.

10. Except for the 5.4L and 6.8L engines, apply a small amount of silicone dielectric compound to the end of the spark plug lead or inside the spark plug boot to prevent sticking, then install the boot to the spark plug and push until it clicks into place. The click may be felt or heard, then gently pull back on the boot to assure proper contact.

11. On the 5.4L and 6.8L engines, slide the coil into the cylinder head and engage the coil onto the spark plug. Tighten the retaining screw and attach the electrical connector.

12. Connect the negative battery cable.



INSPECTION & GAPPING


Check the plugs for deposits and wear. If they are not going to be replaced, clean the plugs thoroughly. Remember that any kind of deposit will decrease the efficiency of the plug. Plugs can be cleaned on a spark plug cleaning machine, which can sometimes be found in service stations, or you can do an acceptable job of cleaning with a stiff brush. If the plugs are cleaned, the electrodes must be filed flat. Use an ignition points file, not an emery board or the like, which will leave deposits. The electrodes must be filed perfectly flat with sharp edges; rounded edges reduce the spark plug voltage by as much as 50%.

Check spark plug gap before installation. The ground electrode (the L-shaped one connected to the body of the plug) must be parallel to the center electrode and the specified size wire gauge (please refer to the Tune-Up Specifications chart for details) must pass between the electrodes with a slight drag.

NEVER adjust the gap on a used platinum type spark plug.

Always check the gap on new plugs as they are not always set correctly at the factory. Do not use a flat feeler gauge when measuring the gap on a used plug, because the reading may be inaccurate. A round-wire type gapping tool is the best way to check the gap. The correct gauge should pass through the electrode gap with a slight drag. If you're in doubt, try one size smaller and one larger. The smaller gauge should go through easily, while the larger one shouldn't go through at all. Wire gapping tools usually have a bending tool attached. Use that to adjust the side electrode until the proper distance is obtained. Absolutely never attempt to bend the center electrode. Also, be careful not to bend the side electrode too far or too often as it may weaken and break off within the engine, requiring removal of the cylinder head to retrieve it.

For a 1998 Ford explorer, if you remove the wheels there is a panel that can be removed behind the rotors that gives you acces to all of the spark plugs. 
Be very careful with this one, apparently these have a ton of troubles with the spark plugs BLOWING from the heads.. apparently the Alum. head where the spark plug screws into only has 3 threads for the plug to seat in, which is not sufficient for holding these plugs in very well.. The reason I am bringing this up is if you try to replace these plugs yourself and if you OVER TIGHTEN the plugs, you can strip the threads very easily. The end result of strippen the threads on the heads is a very expensive head replacement at about  $3000. Some of the spark plugs are also very difficult to reach.
See a full guide here