Update: 6 1/2 years later, the actuator has failed again.
This time I did no wiring diagnosis. I spent more time lifting the front of the truck, placing jack stands--and then after the job was done, removing the jack stands and lowering the vehicle than I spent unscrewing the actuator (1" wrench) and unplugging/re-plugging the single two-wire connector. A guy
could drive the truck so that the passenger-side tires were up on a curb, slide under the vehicle, swap actuators in ten minutes--maybe less. 'Course, all the curbs around here are buried under three feet of snow at this point.
The actuator failed just ahead of what the National Weather Guessing Service is predicting to be a "Historic" blizzard, so this thing had to be fixed and functional ahead of the snowfall.
Yup--the actuator was in fact faulty, Yes, NAPA made good on the "Lifetime" warranty, Yes, the replacement 600-2271 Thermal Actuator is still "Made In USA" according to the label on the box. Yes, the price has gone up considerably--from about $90 in 2012 to $125 plus tax today (except I didn't have to pay, they accepted my warranty return.)
The only extra time I spent on this was to take the photo I missed last time--the actuator plugged in electrically, and fully extended (electrically "live" for about thirty seconds or so--just long enough to pose the photo.) Compare to the photo in the first post to see the range of motion you should have (fully retracted to fully extended.)
I will pull the fill plug on the differential to verify that it's got an appropriate level of axle grease, sometime AFTER I've dealt with the blizzard. Not one drop was spilled changing the actuator this time--so I'm a little concerned that the grease level is low.
NAPA (and I suppose every other parts-house in North America) can get the updated electric-motor actuator (p/n 600-3921) instead of the thermal actuator. The price of the updated actuator is not much higher than the thermal actuator (about $135)--but NAPA wants $80 for the matching wire harness (p/n 600-2944) needed to retro-fit it to these older trucks