The 90 service manual gives a routine to pull RABS codes. When the light comes on, don't turn the truck off before reading the codes. Put it in park, chock both front & rear of the wheels, note whether the red brake light is lit also, then momentarily ground the test connector.
On my 90, the test connector comes out of a large loom under the dash, coming through the firewall, near the parking brake. On mine there are 2 similar connectors in that area, the correct one has only 1 black wire with an orange tracer running to it, & was hidden on top of the loom. The correct one is hanging down on the bottom right of the 90 pic below. 92 has the test plug on the passenger side. See jermil01's 92 pics below.
After you've grounded & ungrounded the wire, the yellow abs light starts to flash. Count the flashes. It said to not count the 1st series, because it may start in the middle. The last flash of the series is a long flash. Then it repeats over & over, until you turn the key off.
Count all of the short flashes & the long one together (So 4 short & 1 long is code #5). If I remember right, it should be a number between 2 & 15 or so. In Steve83's Brakes & Hubs album
, he has the code definitions & other info for RABS-II near the bottom.
There are also more testing instructions in the manual that I can look up after you know what the code is, & whether the red brake light is on or off.