big sherm

Hello, I have a 2006 f150 stx 4wd with the 4.6. yesterday i was cruising down the freeway and my abs light came on. i checked the fluid and it is fine. when i turned the truck back on the light wasn't on but after about 20 more min of driving it came on again. then it did the same thing this morning on my way to work. normally i wouldn't really care if i have abs or not but since i live in the northwest and it rains constantly it is nice to have. some advice on this matter would be greatly appreciated