Please see this video:
http://youtu.be/kvIcsz8YWmc

summary:
sometimes when I close the driver door, the plunger won't go down to lock, and similarly from the outside, the key won't turn in the direction to lock it.

To get around this, I have to pull UP on the plunger until I hear a "CLICK" and then it will go down. The key works the same way.

This is a real hassle for remote locks, the actuator linkage broke off trying to lock it.

Any idea what could be causing this?

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1982 240D 4-speed
2013 Jeep Wrangler Sport - 4.10 e-locker

This may be your issue, inspect the springs on the lock actuator rod, it maybe missing one or both..

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the problem occurs even with the actuator disconnected.

The resistance occurs at the locking mechanism.

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1982 240D 4-speed
2013 Jeep Wrangler Sport - 4.10 e-locker

There is a mechanism that prevents the door from locking when the door is not closed. The problem may lie there.

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