01 Suburban - Only 2WH or 4WL - No 4WA or 4WH.

[5dollar]

So the title says it all. I can shift and engage in to 4Wlow and 2WHigh but when I press the buttons for 4W Auto or 4W High, they just blink and nothing happens. Occasionally I hear noise from XFER case but never engages.

Thoughts on problem?