1997 Blazer LT 4wd won't engage when cold

I have a 1997 Blazer LY with 95k miles.

A couple of winters ago I had a problem getting it to go into 4wd hi or low. When I took it through a car wash where it was warm, it worked and hadn't had a problem since....until this winter. Now, whenever the temp gets in the 20's or below, I can hear the transfer case locking in, but the front hubs won't lock right away. Sometimes I have to be a few miles down the road before they lock. Sometimes if I sit and idle and the engine heat warms things up, the hubs will lock. Most recently, yesterday, I was stuck in a field, no 4wd. Walked to my dads and asked him to pull me out. In the ten min I was gone, the hubs must have locked up and I was able to drive out. (12 inches of snow). No problems above freezing temps.

Anyone have any idea what is causing this? In reading posts, I'm sure it is vacuum related. I also note that my heater doesn't change from defrost to vents right away when I flip the knob. Sometimes takes 2-3 minutes before you hear it flip. Could I have a vacuum leak somewhere?

I'm also temped to buy an aftermarket kit to do this manually. Anyone know of a good one?

Check the vacuum switch on top of the transfer case. If there's fluid in the lines, it's bad and might have contaminated the HVAC vacuum system also.