I tripped my ABS light on my old 2000 F150 and was going to replace it but ended up totally the truck 2 weeks later (not because of that I might add). When I was looking into doing it, I was told the following:
1. I think it is located on the back of the rear differential
2. It's not life threatening and imediately urgent to replace, but will have to be done eventually.
3. I think any autoparts store stocks the replacement sensor (for us in Michigan, try Pep Boys, Autozone or Murrays).
4. I thought they were only like $20-$30 but I could be mistaken, and I was told they are relatively easy to do.
Again, I never got around to it because the truck was gone a week later.
I know there's a few threads on this website that walk you through how to do it, try doing a search. Good Luck!!!