Alright guys as it says 91 s-10 Blazer 5spd 4.3L 4wd high does not work. 4wd low works fine, spins all 4 tires etc. I dont know much about these cars since I work on European makes mostly, someone told me could be a vacuum problem?? Supposedly the 4wd runs partially off vacuum for the change over. Where do I start? Any input is appreciated!

Nate