Looks like you've got a Bosch 2U ABS system
On a typical General Motors application with a Bosch 2U ABS system (1990-94 fullsize rear-wheel drive passenger car), for example, codes can be accessed manually by grounding terminal "H" on the ALDL connector, then counting flashes of the ABS lamp. On these systems, flash codes repeat three times. Codes will stay in memory 50 ignition cycles. But to clear codes, you need a scan tool.
On ABS systems that provide manual flash codes (such as Bosch 2E, 2S, 2U and 3, Kelsey-Hayes RWAL and 4WAL, and ITT Teves Mark 2), you do not need a scan tool but using one reduces the chance of misreading a code.http://www.troublecodes.net/GM/
The ABS brake system has separate diagnostics. Here's how to do the ABS system diagnostic test:
Turn the ign switch to the "ON" position. Jump terminal "H" to terminal "A" of the DLC (AKA ALDL) and count the number of "ANTILOCK" lamp flashes. The terminals must be jumped for a few seconds before the DTC (Diagnostic Trouble Code) will begin to flash. Here's an example: if the lamp flashes twice <pause> once, then that indicates DTC 21 which is listed as "Right Front Wheel Speed Sensor or Circuit Open (Static)".
If the ABS system is working properly then codes 12, 13, 14 or 15 will display depending on whether 2WD/4WD & whether brake pedal is released/applied.
Here's the procedure for clearing ABS codes:
1. Turn the ign switch to the "ON" position.
2. Use a jumper wire to ground DLC terminal "H" to "A" for 2 seconds.
3. Remove the jumper wire for 1 second.
4. Repeat the grounding for 2 seconds.
5. An indication of success is when the "ANTILOCK" and "BRAKE" lamps turn on and then turn off
IMO, start simple
When your up to it, pull and inspect all 3 sensors (one at each front wheel, one at the rear diff), clean off metal filings and goo
Inspect tone rings
Go from there