I have a 1971 Chevy Cheyenne and the doors won't lock freely with the doors closed.  The doors will lock with the doors open or if quite a bit of pressure is applied to the door while it is closed (This doesn't do much good if the window is down to lock the door!).  I have adjusted the door striker in, out, up, and down the scenario is the same in all positions.  Does anyone have any ideas?

There are 'door lock rods' inside the door that tend to get busted, bent, or deformed over time due to rust & lack of grease.
Replacements for these are available for model year specific truck models from sites like
http://www.lmctruck.com/

It may still take some "tweaking" to make these work right for you, even with the new replacement parts. It did when I replaced mine - Chevy '77 K5.
You might want to use the other door ( the one that is working ) - if this is the case for you, as a template of how this mechanism should work. Keep in mind that the functionality will be reversed on the opposite door.
Hope this helps you some.