I had this same problem. I just replaced the sensor underneath the break fluid resiviour but the only place I could find the part is at the stealership. And you should get it fixed soon cuz if the light is on that means your ABS is not working.
trouble shoot before replacing: have you gone over the wiring to discover faulty fuse? broken wire? corroded wire? etc. sounds like a sensor but I would hate to dish out the green for something you dont need
He only has rear wheel ABS.
You most likely have a wiring issue. there is an ABS module under your truck that the plugs in on the bottom. if any of those wires get messed up, the ABS light comes on. I'd make sure that all the wires are good and that there isn't any corrosion.
This happens to my truck all the time. i'd be willing to bet that in heavy rain the ABS light goes off too.