What model and what year is the truck? You'll need a scan tool to check any codes. Are they Wabco or Bendix brakes? Air brakes? Hydraulic brakes? May need to check the VSS Sensor on top of the rear differential, be sure it's plug in, check adjustment, etc. If it's air brakes, it may be oil on a wheel censor or one may be out of adjustment. Loose wheel bearing will make the light come on. Tire pressure correct all the way around? Driven in any standing water lately??? Are your backing plates still in place? Water splashing on the shoes making the shoes swell. This will throw the ABS for a loop until theres enough heat to evaporate the water. Is the truck under warranty, if so see your local Dealer.

Check your brake fluid reservoir cap.  If is miss seated it will throw the ABS light on.