Tools Needed:  Grinder and Welder
This is not an easy repair.  The power seat assembly was not made to disassemble.  You will have to grind two spots to disassemble and then reweld for assembly.
 
2002 - 2005 Mercury Mountaineer Forward Backward adjustment steel gear kit.  This kit is for one seat repair for the forward backward movement due to broken gears.  The kit comes with two steel gears mounted on two custom machined shafts.  A lifetime warranty against the steel gear breaking comes with this kit.  You will have to reuse the factory small worm gear.
 
This kit also comes with a new front thrust washer, a small bottle of grease, as well as detailed instructions with colored photos. You will reuse your rear thrust disc and other mounting hardware.