4-Wheel Drive Won't Engage

I have a 2007 Tacoma, V6 automatic, 138K, and switch 4wd . The 4wd will no longer engage. Looking for a good place to start with diagnosing the issue. Details are as follows:

About a year ago I turned the switch to engage 4hi and it went just fine. Shortly after I disengaged the 4wd and continued on the trail. A few minutes later I went to switch into 4hi and nothing happened. No sounds, no indicator light, nothing in 4hi or 4lo. It did not engage.

The next day as a test I went outside and switched into both 4hi and 4lo with no problems observed. I then disengaged the switch drove around the yard and then tried to lock the 4wd back in, I got the same problem as stated in the paragraph above. I continued this test for several days with the same results.

Due to the 4wd issue I was experiencing at the time, I went trail riding one day, locked into 4hi and left it locked in for the duration of the ride. Once I disengaged the 4wd I decided to try to lock the 4wd back in. Expecting my usual result of nothing. To my surprise it locked in fine, to both 4hi and 4lo. After that day I never experienced any more problems until now.

A few weeks back I noticed every time I start my truck the 4wd dash indicator light will flash one time only. Didn't think much of it until I tried to lock into 4wd and it doesn't engage. Nothing happens at all. It is similar to my previous problem, but now it won't even lock in once.

Any suggestion on where to start?

And I know there are previous post of similar issues, however I couldn't find any recent post on the issue. If you feel the need to be a smart a** about not doing my research before posting, at least leave some advice related to my issue as well.

Thanks in advance.

 

Actuator? @Sacrifice

 

not sure. mine would engage but the light would flash constantly therefore the ecu didnt know if it was in 4x4 or not. which meant i couldnt change to 4lo or back to 2hi. thats how i figured out it was the actuator itself.