How do I remove the rear upright portion of the seat on my 2000 ford f150 lariat 4x4 truck?

Check this procedure for Ford models (click over images for zoom)....

Rear Bench Seat Cushion (Supercab)

Fig. 7 Rear seat cushion installation-F series Super Cab

1. Remove the seat cushion retaining screws.
   
2. Lift the seat cushion and feed the seat belt through the cushion.
   
3. Lift the cushion up ensuring the retaining hooks are free and remove the cushion.

1. Place the cushion into place, place the seat belts through the cushion and engage the retaining hooks.
   
2. Tighten the retaining screws to 19-25 ft. lbs. (26-34 Nm).

Fig. 8 Rear seat back installation-F series Super Cab

1. Remove the seat cushion.
   
2. Remove the seat back bottom retaining screws.
   
3. Lift the seat back and remove it from the vehicle.
   

1. Place the seat cushion into place ensuring the retainers on the top of the seat back are engaged.
   
2. tighten the retaining screws to 19-32 ft. lbs. (25-44 Nm).

Fig. 9 Rear bench seat and track installation-F-350 Crew Cab

1. Remove the seat track-to-floor pan bolts and lift the seat and track out of the truck.
   
2. Installation is the reverse of removal. Apply sealer to the area of the bolt holes. Tighten the bolts to 18-32 ft. lbs. (25-44 Nm).

Fig. 10 Side-mounted folding rear seats-F series Super Cab

1. Remove the seat bracket-to-floor screw and washer assemblies.
   
2. Remove the seat assembly.
   
3. Remove the bumper screws-to-retainer.
   
4. Remove the bumper.
   
5. Remove the trim panel retaining screws.
   
6. Remove the seat assembly.
   
7. Tighten the bolts to 22-30 ft. lbs. (30-40 Nm).

1. Release the rear seat back latch striker and put the rear seat back frame in the fold down position.
   
2. Remove the seat belts.
   
3. Remove the retaining screws attaching the rear seat back cover to the rear seat back frame.
   
4. Lift up the seat back cover and remove it from the vehicle.
   
5. Remove the panel and support assembly-to-floor retaining screws.
   
6. Pull the rear seat back release latch and remove the rear seat cushion frame and rear seat cushion cover.
   
7. Move the seat cushion so that the bottom panel is toward the front.
   
8. Remove the retaining screws to the bottom panel and support assembly.
   
9. Feed the strap-to-rear seat back release latch through the rear seat cushion pad.
   
10. Remove the rear seat cushion.
    
11. The installation is the reverse of the removal.