There's a couple of things that can cause your rear brakes to lock up. The first and most obvious is that it's time for a brake job. If the drums are out of round or if the shoes are worn they can lock up like that.
Another possible cause is a wheel cylinder leaking. Brake fluid on the linings will cause the brakes to lock up. Pull the tire and drum and take a look.
Your '89 should have rear anti-lock. There's no proportioning valve back there but the abs electro-hydraulic valve controls the balance between front and rear brake pressure. When braking hard, you should not let off the pedal if you have ABS. The ABS system won't work if you let off the pedal when it goes "squishy."
Good luck!
Brian
EDIT: Fixed typos!