4WD not engaging

We recently purchased a 1997 Blazer 4WD but the 4WD does not seem to be engaging. It has the push button instead of the stick shift and the buttons seem to be functioning.

If you push the 4wd button while driving you can feel (and hear) the engine change slightly but it doesn't engage.

Any ideas on what could be causing this problem?

http://www.automotiveforums.com/vbul...d.php?t=572914 (http://www.automotiveforums.com/vbulletin/showthread.php?t=572914)