Just remembered something last time I was messing with my blazer. The transfer case switch...curious, i pulled off the hoses...to find fluid dripping out. Didnt know if it was normal so put the hoses back on. Decided tonight to do a quick search, and found out...it is transmission fluid in my vacuum system...(not good)...but anyways, this website should explain maybe why your 4x4 maybe isnt working.

GMC Sonoma & Chevy S-10 Transfer Case Vacuum Switch


I will be ordering a new switch, and then hopefully soon, my 4x4 will work.