TS 4Runner 4WD light, focusing on ADD Acurator

My truck has a problem to shift to 4WD mode. The 4WD light does not turn on most of the times I tried. After google, found some related posts with similar problems all pointed to ADD actuator.
This is recorded in the post at t4r.org.

The Automatic Disconnecting Differential (ADD) system is explained in this article.
The manual I had did have several diagnose pages on related topics. I need to go over them one by one...