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Does the ABS ever activate while you are slowing down (shudder and pulse the brakes)?
Have you ever had the vehicle scanned for trouble codes in the ABS system?
If the abs isn't false activating then the most likely cause of the light would be defective wheel speed sensors or worn tone rings (the metal part of the hub that the wheel speed sensor picks up the signal from). It's nearly impossible to tell for sure what it could be without having the truck scanned for codes though. The ABS light could come on for hundreds of reasons.
Do you have a multi-meter? If so you could run a crude test on each wheel speed sensor that would tell you whether the sensors are working.
Please let me know if you would still like help with this problem.