Did you replace the battery for this reason or prior ?
Battery cable sets on these fail or connections fail long before the starter does. It is very common on that vehicle to have a voltage drop to the starter, due to interior corrosion on the cable, stuff you can not see. You need a volt meter to check just how much voltage is getting to the starter. If it is not getting the 12v it needs it will not engage. Take 2 aspirins before getting a price on cable set from Ford, the after market sets do not fit or work.