I have a 2003 chevy blazer LS, 2-door 4wd w/

I have a 2003 chevy blazer LS, 2-door 4wd w/ vortec engine. The 4wd is activated by a 3-button switch on the dash (2wd-hi, 4wd-hi, 4wd-lo). The problem is I can't get it to go into 4wd-hi no matter what I do. When I shift into 2wd-hi or 4wd-lo the switches light up momentarily and I can hear a clicking sound from under the dash behind the glove box, and then the transfer case goes into these gears just fine. But when I punch the 4wd-hi button, nothing happens. No light, no clicking noise from under the dash -- nothing, it just won't go into gear, it stays in 2wd-hi. Help!