we had our first tiny little snow last night so I figured i'd go out and play. My 4wd would not engage at first lights would just blink. You can hear it trying to engage in the transfer case but it's not happening. At one point in time it actually did work in auto 4wd and stayed in for the rest of the drive. This morning it was back in 2wd and went on its own. Any ideas?

 

Have you checked the connections at the dash switch, TCCM, and encoder motor for corrosion?

 

Well an update but nothing so far. I looked over all the vac lines and haven't found anything leaking. I took the battery tray out and looked over the actuator and everything seems fine. I'm also having a problem I think I put in here about my vents only blowing out the front and defrost at the same time...no floor vent. I took the hvac control box out of the front dash last night didn't see anything out of the ordinary. Also looked under/behind the glove box for any leaks and didn't seem to find anything. One thing to note when I try to put it in 4wd I can hear it trying to engage in the transfer case and I can hear the TCCM box click but the lights just flash. I swapped out the 4wd button box in the dash and also checked/cleaned the connection on the encoder motor

I did get a small glimpse of the Vac Reservoir but it looks impossible to access it unless the ABS is removed off the vehicle or the front fender is taken off.

 

You have a vacuum leak somewhere, going to have to track it down and fix it

 

Update.
It was the front drive axle vacuum actuator solenoid GM Part # 15165891
They are like $55 at GM. Napa has them for just about $30

 

About the issue you are having with the vents not blowing correctly. I found in my 92' Blazer that the clip on the flapper door down by the hump for the trans was disconnected from the rod. I had to bend it back and was a tight fit to get my hands in there but managed too and works fine now. Hope this helps.