GM reports that all of its 1998-2010 mid-size and full-size trucks are prone to illuminating the ABS light. According to the car maker, the problem is often erratic or intermittent, with trouble codes C0055, C0235, C0236 or C0237 logged in the ECBM when the condition occurs. A problem in one or both of the rear wheel speed sensor circuits is a likely cause.
To make a definitive diagnosis, hook up your scanner and check for codes in all the other modules of the truck. If you find any, take care of these first. If the only codes stored are the ones in the ECBM, that confirms that the rear speed sensor circuit(s) are compromised. Check the wiring from each sensor back to the module, and repair as necessary. Depending on the vehicle model and year, these circuits are numbered 821 and 822 or 400 and 401 on a wiring schematic.