I start the truck, and the switch light flashes initially like it always has, but now the green light comes on steadily, and after manipulating the switch I can get the lights to shut off. Then when I proceed down the road, the light will come back on (green), and at 45-50 mph you can hear that the transfer case and 4wd have engaged on their own,,,,,,,,Ghost, or vacuum hose?

 

I would lean towards vac bud.
I was just reading a very informative thread on the vac system . Ill post the link here for you.

 

https://blazerforum.com/forum/showth...605#post547605


Good luck...sry about the mobile format...I'm on my phone!

 

the switch may be bad. just unplug the switch. then try to drive it. if it still engages might be the tccm transfer case control modual is bad. or the encoder motor is stuck in 4wd. for a temp fix unplug the vacuum line from the actuator under the battery tray and plug it with a bolt or screw.

 

Update. It was an Old Man issue. Apparently I bumped the 4wd switch while toggling the rear wiper switch next to it. Finally discovered what the problem was after checking all the hoses-switches etc. It's just great getting old and forgetful,,,,,,,,,,,,,,,