I took the hatch panel off per R50JR's instructions this morning (I was waiting for cooler weather, as my shop isn't air conditioned). It came apart pretty easily with a minimum of broken panel clips, and I found that lock actuator isn't working. When I removed the bar from the actuator (that goes from the actuator to the latch) and pushed over on it manually, the hatch opens normally.
I priced the actuators thinking they'd be cheap, like a passenger door actuator. Wrong - They're $250 or so. I decided to do a little more looking online and found this thread:
https://forums.nicoclub.com/lift-rear-gate-latch-problem-parts-help-t455161.html
I took apart the actuator and found the same broken gear tooth as Gcohen6 describes. I removed the broken tooth, reassembled the actuator, and reinstalled in the the liftgate. It works now - the broken tooth must have jammed up the actuator. I don't know if it will work indefinitely, but for now it's functional and thanks to R50JR's instructions getting the panel off to get to the actuator isn't hard, so I'll replace the actuator if this one jams up again.
Thanks again for everyone's help.