1998 Ford Expedition engine runs rough during acceluration.

IF the check engine light is on, you need to scan it again to see if the problem has changed. The coil pack may be weak or I have actally seen the rubber boot on the coil pack burn through allowing the spark to short out on the inside of the valve cover resulting in a miss. Did you check the rubber boots for any signs of this? Usually there will be a gray spot where the spark arks through. Give that a check to see if that is the problem. Believe it or not, it is more common than many know. Most mechanics will say you need a new coil pack when only the boot is bad.