ABS warning light on 2000 F150

I have a constant ABS warning light on my truck. The brake fluid level is ok. I have been told the most common fault is the rear sensor in the differential. Is this usually it. If so, is there anything special to know to test, or to change it out. I have been driving it this way for a while and finally deceided to have a go at it.

I removed the rear abs sensor. I put a ohm meter on the two pins and it tested open. I didn't know what it was to test. Some devices are open until activated and some are closed until activated. So which was it? The local NAPA had a sensor for the lowly sum of $15 so I took the chance and got one. I put the meter across the new one and it had continuity. The old one was bad. I installed it and the abs light went off. So you know you have a bad rear abs sensor on a Ford when it tests open with your ohm meter. And that's the lesson for the day!