I'm not sure about the way the ABS gets the signal, same with the ECU. I think a lot of Fords get the speed signal from the instrument cluster, since the instrument cluster also has the circuitry for speedo corrections. So the problem may be somewhere after the signal has been processed. You can get the ABS codes pulled and see what it is complaining about. Also pull the engine code, and see what it says. You should never leave a CEL on, mostly because if you have the light on for some minor thing, and then something major happens, you will not know about it right away because the light is always on. Its kinda like having a smoke detector that just randomly goes off, pretty soon it just looses all credibility.

Let us know what the codes are, there is a sticky that tells you how to do it.
http://fords.kilonet.org/ken00/EECIVQT.pdf