The likely culprit is one of the ABS wheel-speed sensors, a pinky-size part that detects the teeth of the tone ring. The tone ring looks like a gear and hooks to the axle hub; the wheel-speed sensor "sees" the teeth and can determine how fast the wheel is moving. If the sensor is dirty, out of alignment, broken, or not putting out enough electrical signal, the computer can't determine what that wheel is doing, so it puts up the warning light. A scan tool might even tell you which sensor is acting up. You might need to remove the caliper and rotor to access the sensors to inspect and clean each one, so it's going to be an afternoon job.
If you find anything amiss, such as a broken or worn wire harness or sensor, replace without trying to salvage things. Put everything back and remove the positive battery cable for a half-hour to reset the computer. Drive around and see if the warning comes back. If it does, consider taking the truck to a reputable brake shop. It might sting the pocketbook a bit, but so will failing to stop fast enough in an emergency.
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