ABS ANTI LOCK light on

I have a 96 taHOE and last week the anti lock light has been coming on. At first it would just stay on a few minutes now it stays on until the car is off. I checked my brake fluid and its perfect. What could be the prob?

Have a look here, http://www.automotiveforums.com/vbulletin/search.php?searchid=912128