More than likely, someone has disconnected the vacuum line in the engine bay that feeds the door lock system. Once you get a mityvac, there is good information here to help you troubleshoot vacuum leaks. Its not that tricky, just takes some time.
As for the door that won't open, I think you can remove the rear door inner panel without having to open the door. Dieselgiant has a good pictorial for removing a door panel
here.
As stated earlier, a linkage may have come loose, and you need to get to the inside of the door to check it out.