My car won't lock from the outside?

I have a 99 Ford Explorer and you can open it from the passenger side door even if it's locked. The other doors are alright. (Except for rear drivers side that doesn't open at all) The alarm doesn't even go off if you open it while it's locked. Can I fix it myself or should I take it in?

If you are not good with your hands take it to an alarm/stereo shop. They will charge you way less then an automotive shop. Probably under $50 to fix plus parts (if any) could be your door actuator is bad. You can get that online for cheap. If the alarm/stereo shop has one for under $15 just get it there.