It is funny you should ask.
I am changing mine this weekend. ($30 at the dealer).
First I am going to try to clean it as I am told by others.
If that does not work, then I will change it.
I have not seen it, but I am told it is located on top of the rear diff.
Clean the area around it first so no dirt falls in.
There is 1 screw to take out and then gently pull it out or pry it up.
ALWAYS ALWAYS ALWAYS make sure your socket/wrench is the right size or you will strip the head. Even if you have to lightly tap ot on.
My 99 F150 XLT 4X4 ABS is kicking in at 10KM/HR on pavement.
No check engine codes or ABS warning light come on at all.
If this does not work, then I will have to take it in as the expensive ABS sensors ($230 at the dealer each) on the front bearing are the next thing to try. (The whole front hub with ABS sensor on it is $330)
I am told it may be a ground issue or even some other ABS parts.
Right now I just pulled the #25 fuse out from under the dash for the ABS.
Breaking still work great but you have no ABS for winter driving.