1999 Ford Explorer door locks

I have a 99 ford explorer xlt with over 200,000 miles runs good with few unexpected problems, but one is, the passenger side door only unlocks with the remote but won't lock, the same with the door control. The remote will only unlock the doors but will only sound the horn without locking the doors when it is used, and when I put the truck in drive the doors no longer auto lock. What could be the problem with my locks.