My Blazer will not go into 4-wheel drive, it makes a noise trying to go into 2WD on 2002 Chevrolet Blazer

It sounds like it isn't totally in 2WD but it definitely will not go into 4WD at all. It makes a loud "humming" like it is trying to go into 2 WD.

Vacuum switch problem!!!!!
I thought it was a hub bearing or universal joint from the way it vibrated and sounded. I had the left front hub bearing replaced last August and the right front one replaced in June but it still makes the noise and won't go into 4WD. The mechanic said the vacuum switch was okay?