Your truck, I would think, has RABS and not full ABS. So the light on means that the RABS computer has detected a fault and disabled the system. You will probably not notice any difference in braking as the RABS is not much good anyway.
However check the simple things first, check that the rear brake lights are working properly and check that the brake fluid level is good.
Can also be an idea to remove the sensor from the rear differential housing and clean it, then refit.
You do not need a code reader for the RABS. There is a diagnostic connector behind the glovebox that you ground momentarily and the dash light will blink out any error codes.
However try the simple checks first.