'89 would have been very early for Mopar ABS, and they had issues.
check for clean sensors first. But I had a '90 Dakota that I bought new, and I think it was recalled 3 times for rear wheel ABS problems. The first was the harness connectors under the truck were not waterproof, and they were upgraded to a different style. The second and third trips back were because the ABS "brain" failed (under the passenger side dash)was upgraded to a new part number each time. You'll need to find a shop manual, you'd install a temporary ground lead on the brain and it would give you a code. The brain upgrades may have just been warranty work, not a recall. Since you have a Ram, not all of this may apply, but it might since they were about the same time frame.
Thinking back, you may not find any sensors to clean, because your tone rings are probably not at the rear wheels. There is probably an electrical harness running to the top of your differential, and the sensor is probably inside the differential, reading a tone ring on the side of the ring gear. The sensor may be able to be replaced from the outside without opening the rear differential, just a guess.
