You will need to get the ABS codes pulled.. Your standard OBD II reader probably wont do it. (but if you want to try your luck, most Autozones will ready your codes for free) Manufacturers have been fighting with the courts on trying to keep their diag codes private so you have to go to their shops. THey lost on the engine/emission codes....giving way to the birth of $40 code readers. However transmission, ABS, body/powertrain codes they were able to keep a secret. There was recently antoher court ruling saying they have to give out these codes too, but ive yet to see any results of that yet.
I'd starty with pulling off your wheels that have ABS on them. THere will be a speed sensor on each wheel that has ABS (2 or 4). Gently clean it with a paintbrush. (I dont know if stronger cleaners will hurt it or not... if you wanna try go with something waterbased). Dirty sensors is a common problem with abs systems. THe other common issue is your pump... expensive.
Any time your ABS light is on, it is disabled. I had a car (dodge stratus) that had a failing pump system and the abs would go off whenever it felt like it.. This is a dangerous condition as you may not be able to stop for a light. If in doubt, pull one of your ABS fuses.. (make sure it doesnt kill other systems in your truck). This will put the light on all the time, making the system safer till you can find the problem.
a quick google search poped this up. .may be worth a look: