'97 4x4

Hello people, I have a 1997 Chevy silverado 4x4 ext. cab . My 4x4 doesn't seem to be engaging . Where should I start first ? It has the 4x4 shifter on the floor, not push buttons. By engaging I mean , I can shift on the floor to all positions . But only when I'm in Lo do I feel any change , which is more than likely the transfer case itself being in low range . I don't feel any like grabbing while turn slowly on grass or pavement.