Ford f150 crewcab - back seat won't lock in place in sitting position.

If you mean the seat portion slides back and forth check under the seat. A pawl should engage a stationary saw-tooth mechanism. The pawl may be stuck in the up position.

Will check that area. Mainly, though, the folding seat is great for the amount of room created on the floor in the back seat area, but when I would like to use the seats as seats instead of folding to the back of the cab, the bottom (sit-on) portion of the seat will not lock into that position so the seat could fold (like a folding chair) if in accident or rough roads or turns.

Sorry, I misunderstood. The mechanism which holds the seat in the down position is located at the pivot point, controlled by the lever on the side of the seat. It's spring loaded and operates in basically the same way as I indicated above. You may have to remove the handle/lever to access the mechanism to see what is hanging up. You may be able to see the problem with the seat up and looking at the mechanism from underneath. Moving the front seat all the way forward may give you more room to access the area.

Another update. I have a 2010 crewcab. The back seat locks in the down position. On some models, and your's may be one of them in the supercrew configuration, the seat is designed to only lock in the up position with gravity holding the seat in the down position. The owner's manual or net may be of assistance here.