Engaged the rear locker the other day to back out of my spot at home. We had about 7 or 8 inches of icy sleet/snow mix. Hit the button, it locked normally, I backed out of the spot. Got to the street and turned the locker off and. nothing happened. The lock light stayed on and steady. I tried turning it off and on, no change at all. I had to go to work, it's not far, so I just drove it with the rear locked. Pushing straight in corners, chirping tires on tighter turns, ah well.
After a couple of days the weather turned a bit less miserable so I was able to do some diagnostication on it.
No fuses blown, there are potentially 2 involved, one for the gauges that will prevent the light from lighting, clearly not an issue as the 'lock' light stays on (and the ABS off). And the '4WD' fuse, also in fine shape.
I briefly crawled under the rear to double check that the wires were still plugged into the actuator, all good under there.
Then I pulled the driver kick panel and did some basic multimeter tests at the locker ecu plug, all seemed good (although, derp, I was a wee bit confused when the transfer case wire wasn't doing what it was supposed to do, but then I recalled I did the gray wire mod on it 15 years ago, d'oh). Nothing seemed wrong here, other than what I couldn't easily test.
So I pulled the carpet up in the LR area, consulted a wiring diagram, and used a spare 12V battery to apply current directly to the M1/M2 pins. The motor locks or unlocks based on which way around the 12V+/GND is applied to them. Applied it one way, nothing. Applied it the other way, could clearly hear the actuator moving. So I got a jack, lifted one rear tire, and applying the current back and forth the axle would lock and unlock normally. Interesting. Left it 'manually' unlocked, and plugged the rear axle harness back in, and tried the dash locker button. And it locked normally (well mostly, it seemed a bit weaker?). Tried to unlock it, absolutely nothing. Looking carefully, I could see the dash lights dim every so slightly when I'd engage the locker via the button, for about 4 or 5 seconds (the amount of time the locker ECU will attempt to run the motor), then they'd return to normal. Nothing at all happened when I tried to unlock it.
So I unplugged it and manually unlocked it again with the spare battery. Plugged it back in and this time, trying to lock it via the dash button it made a little noise, but didn't fully lock The ECU not sending it enough current to actuate? Unplugged and locked/unlocked it several times via the spare battery, it worked normally.
Seems like enough suspicion on the locker ECU at this point, so I got on the internets and found one for $26 shipped, ordered it. That's an easy swap to see if anything changes.