The first step is to pull the ABS codes. That will give us a starting point for trouble shooting.
This model supports flash codes, you will need to jump across terminal A and H of the ALDL connector (under the drivers side of the dash). It may take about 30 seconds before the ABS light starts flashing codes.
Count the flashes to get the code, they repeat 3 times if I remember correctly.
Edit your question to include the codes you get, I have listed the codes below for reference, but will continue to help you once you let me know what the codes are.
The list of codes follow
Code 21-RF speed sensor or circuit open
Code 22-Missing RF speed signal
Code 23-Erratic RF speed sensor
Code 25-LF speed sensor or circuit open
Code 26-Missing LF speed signal
Code 27-Erratic LF speed sensor
Code 29-Simultaneous drop out of front speed sensors
Code 31-RR speed sensor or circuit open
Code 32-Missing RR speed signal
Code 33-Erratic RR speed signal
Code 35-VSS or LR speed sensor or circuit open
Code 36-Missing VSS or LR speed sensor or circuit open
Code 37-Erratic VSS or LR speed sensor signal
Code 38-Wheel speed error
Code 41-54-Control valves
Code 61-63-Reset switches
Code 65-66-Open or shorted pump motor relay
Code 67-Open motor circuit or shorted BPMV output
Code 68-Locked motor or shorted motor circuit
Code 71-74-Memory errors
Code 81-Brake switch circuit shorted or open
Code 86-Shorted anti-lock indicator lamp
Code 88-Shorted brake warning lamp