There really isn't a "tune-up" on a distributorless ignition, sequential fuel injected car engine such as the SHO's. You are referring to replacing the spark plugs which is only one component of the 60K preventative maintenance service. You should look in the owners manual or use the Forum's SEARCH feature to get the list of items that should be serviced or replaced, depending on the mileage and condition of your car.
You can replace the rear bank spark plugs by removing the vacuum hose on the passenger side and the 12mm bolt that secures the vacuum accumulator assembly in the center of the rear intake plenum. You will then need 9" of extensions on your socket wrench and 5/8" spark plug socket. Use ONLY the Motorcraft AGSP32PP plugs gapped to .042"-.046" (I recommmend .042" since the gap never gets smaller), 17 lb-ft of torque to tighten them, anti-seize on the plug's threads and dielectric grease inside the plug boots. Be gentle with the wires as you can damage the conductor or insulator if you bend them too tightly.
If you find oil in the plug wells, you must clean it and any other loose debris out before you even think of removing the plugs. You don't want any of that stuff to fall into the cylinders and score your cylinder walls.
