Sounds like the vacuum switch is getting stuck. Just remove it and clean it out. The switch is located on your transfer case and you'll see 3 hoses going to it.
Btw thanks to Kyle for the above picture. I'm currently using this to clean out my vacuum switch.
And if you are still having problems, then remove your battery tray and check the Vacuum Actuator, they are known to go bad as well.
Dave- 01 blazer 4x4, 6" Superlift Kit,2" Body Lift,15x8 on 32's and 20"x9" on 33's,New Jersey