If your truck has 4 wheel ABS start here
-measure all four tire circumferences, they should be within 1/2" of eachother
-remove the rear sensor from the diff and inspect for metal shavings on it or damage
-remove both front wheel speed sensors and look in the hole as the reluctor ring turns, and inspect for any chipped or missing teeth, or cracks....odds are replacing both front wheel bearings will fix this
does it activate more if you are turning one way or another?