Well, it could be any number of things, but actually has nothing to do with your brakes, really. The ABS system is totally independent of your basic braking system.
Start by disconnecting, inspecting, and cleaning all the WSS (wheel speed sensor) connectors behind each hub under the car. They're the only electrical connections on the hubs.
After you do that, let us know what the results are, either positive or negative, and we'll take it from there. Unfortunately, this is a very expensive system to repair if the above advice doesn't work. You may end up learning to live without the ABS.
I'm moving this to the 92-99 mechanical forum for better visibility.