I will bet you a million dollars you have a vacuum leak somewhere, I just fixed my no 4x4 HI problem after finding a disconnected vacuum hose on the windshield washer fluid resivor. It's weird that prevented me from getting 4wd HI but it did and since you have tryed everything else check and replace any lines that are bad and make sure everything is connected.

 

Found this thread and how to and worked for me thanks who ever posted it and were ever I found it lol I was lost on here and stumbled across it lol https://blazerforum.com/forum/tech-articles-diy-29/how-homemade-posi-lock-cable-system-31138/ may have to copy and paste I'm on my cell so can't do much but works great for me been involved with this s10 for 4.. yrs and 4wdI never work and I hit all the spots I've always got stuck and walked all over them paybacks a bitch ill get videos and pix up soon

 

It was a vacuum leak the hose was ran to nothing likea vent tube who ever ran it when they replaced it cause the actuator looks new and hooked the vacuum up and 4wd

 

90% of all 4x4 engagement problems are for that reason. Glad you found it.

 

I pretty much found every vacuum hose in my truck before I found the obsceure hose disconnected from the washer fluid resivor. Glad you got it figured out, it's nice to finally figure something out that has been bothering you for so long.

 

Just a note since it just happened to me, if you have made sure all of your hoses are connected and you are still getting a leak, I was getting a leak on the line the runs from the engine bay to the vacuum switch and it turns out I had to reset my transfer case motor and finally I have 4wd again. So if you are sure you have no vacuum leaks then try resetting the transfer case motor by unpluging it buy unbolting it and then turn the keys on and off a few times I believe.

 

my broblem was o ring gone on vacuum switch top of transfer case easy to detect as switch turned by hand