Is your engine the 4.6, with the two-coil EDIS system, or the 5.4 with eight individual coil packs each serving a cylinder? If you need more info, you should hit www.ford-trucks.com and check out their F-150 or Expedition message boards. Hopefully, if it's the 5.4, the coil in question is located on one of the more forward plugs, as the rearmost cylinders are a superb pain in the ass to deal with from an accessibility perspective.

BTW, on 5.4's, the #4 coil pack (passenger side allll the way back) is notorious for shorting out, particularly on the earliest Expeditions, due to leaky heater hoses dripping antifreeze onto the coil. Later heater hoses (including current replacement hoses from Ford) are better designed and less likely to leak.