2000 Chevy Blazer 2000 Blazer 4x4 will not engage

Hello, I just recently bought my blazer and the 4x4 does not work. When i push either the auto, 4hi or 4lo I can here the transfer case go into gear but when i put it up on my lift i see that the front does not spin. So obviously the front diff is not doing its job, are there any parts i should check ie. the vacuum accuator, cables or what not. And if so what am i looking for?

Thanks