Some closure here. I took the Journey to the dealer and they found a broken wire in the wiring harness. I was told that a new harness would be about $190 plus an hour's labour at about $130 per hour, plug taxes. "shop supplies", etc. I was told that the harness couldn't be fixed as the break is in a "sealed" portion of the harness.
Instead of trying to open the up harness, I just fished a new wire through the rubber "hose" between the hatch and the vehicle, and spliced it to bypass the broken one. While I was working on the issue, the tailgate interior light failed altogether (rather than flickering while opening), so I fished a couple more wires--one for the light and a spare if anything else fails.
Interestingly enough, when I was looking at the wiring harness, I saw that the insulation for the tailgate interior light wire was chafed and the copper was exposed--probably from the factory as it was being installed--so that was probably the point of failure for it. I imagine that the tailgate lock switch suffered the same fate even though I didn't see any visible damage to it.
