There is a technical service bulletin out for this concern:
DRIVERS FRONT POWER SEAT WILL NOT MOVE FORWARD OR REARWARD
This article supersedes TSB 05-9-4 to update the part information and Service Procedure.
ISSUESome 2003-2005 Expedition/Navigator vehicles built before 12/06/2004 with memory seats may exhibit the driver's power seat binding in the full rearward position and becoming inoperative in forward and reverse. This may be due to the horizontal seat motor and memory seat position sensor (potentiometer) on vehicles built before 10/16/2003, or due to the potentiometer (only) on vehicles built between 10/16/2003 and 12/05/2004.
ACTIONIf the vehicle was built before 10/16/2003, inspect the horizontal motor bracket under the seat. If the bracket has bends on either side of the seat motor, install a horizontal seat motor kit. If the vehicle already has a straight bracket, or if the vehicle was built between 10/16/2003 and 12/05/2004, install a seat potentiometer kit only. Refer to the following Service Procedure.
NOTE WHEN INSTALLING A MOTOR KIT, THE SEAT TRACK FRONT COVER MUST ALSO BE REPLACED. THE ORIGINAL SEAT TRACK FRONT COVER WILL NOT FIT ON THE NEWER MOTOR ASSEMBLY BEING INSTALLED.
NOTE FOR 2003 NAVIGATOR VEHICLES EQUIPPED WITH A BLACK INTERIOR, ORDER THE DOVE GRAY SEAT TRACK FRONT COVER AND PAINT TO MATCH (BLACK INK).
REMOVAL - SEAT MOTOR ASSEMBLY OR POTENTIOMETER
Basically you would have to replace a potentiometer and some cables on the front of the seat motor.
Did you try reprogramming the memory seat settings?