So the lock won't go up and down on the driver's side door, thus you can't lock the door.

I took off the door panel and looked inside. There is a rod that connects the lock assembly that you can see, to some sort of cylinder which has an electrical connector on the bottom (for the power door lock).

I tried grabbing the rod with pliers and forcing it myself, but it wouldn't budge. I tried rapping the cylinder with a hammer while jiggling the rod, and it still wouldn't budge.

The cylinder is riveted or welded into the door, so I don't think I can take it out for replacement.

Any ideas on how I can get the door to lock?

 

Not familiar with '91 models, but later models actually have TWO rods: one from the button to the latch mechanism in the door, and one from the power actuator to the latch mechanism. The latch is positioned obviously on the rear edge of the door, about 1/2 way between the motor actuator and the manual lock button.

It's possible that the latch had jammed, but it's also possible that the motor drive has failed. Disconnecting the motor drive mechanically might free it up so you can lock it manually.

It's possible that 91's have the motor integrated into the latch somehow. In later Rangers the connector on the latch is for the dome light, not for a motor.

Sorry to not be specific, but I have no info here to look at. I have a service manual to a 1992 Ranger at home, but I'm not sure what it tells you overall about that system.