I had a 1999 and a 2003 blazer and both of them when the 4 wheal drive stopped working I noticed that the heater did to it would not switch from defrost to floor to vent. And both times it was the vacuum switch on top of the transfer case. You can see it best from front drive shaft looking on top of the transfer case there are three vacuum lines plugging into the vacuum switch. A little tough to get to. A new switch coast about $30 it fixed both of mine.

 

Leaking vacuum lines, anywhere in the vacuum system, will also cause this. The t-case switch can be working fine, but a broken or disconnected vacuum line will cause what you describe.

 

yes I also had a vacuum leak on both blazers i owned over by the breaking ABS system there is something in the drivers side fender very hard to see that a vacuum line goes to. I dont know what it is I had a hard time getting to it

 

It's the vacuum reservoir. Stores vacuum for times when the engine isn't producing enough.