could be a crooked worn pad, or something caused scoring and the pad schamfers to the side when brakeing, also if its a light noist could be metal shavings built up on the toner ring or abs sensor

 

My truck is doing EXACTLY the same thing. 2005 with 75,000 miles. I've done quite a bit of research over the past couple days, and the most popular problem appears to be the hub. More specifically, if I understand correctly the tone ring separates from the hub, which prevents the ABS sensor from getting any kind of accurate measurement of wheel speed. I plan on taking mine in to get it coded to see what the computer says is wrong, but again that's what I've found. There's a thread with pictures on this forum that shows places you can check to make sure wires aren't broken, no dirt is in the hub or around the sensor, and even shows how you can try to use a screwdriver to tap the sensor back in place if it has moved a little. Are you only getting the ABS light, or are you getting the brake warning light as well? I'm getting both lights at the same time.

 

This happened to me about two months ago it happened for a little then would stop and then happen again then it would go away again really weird. When I did my upper b/j flip I
took a closer look at it i didnt really do anything just cleaned the rotors and everything real good and checked the abs sensor wiring harness and I haven't had any issues to date. There is a how to on this: www.355nation.net/forum/how-chassis-suspension/31860-how-diagnose-repair-abs-fault.html Hope this helps.

 

Rotors may be warped... check them out.