The technical term for these rings is "reluctor ring" because it uses something called reluctance to count the teeth as the joint or bearing revolves. Simply put, it is resistance in a magnetic field rather than an electric circuit. As the teeth pass the reluctor magnet a pulse is generated. Crankshaft position sensors use a similar system.
What usually happens is they rust and break so the count process no longer works correctly, especially at low speeds. This happened to me with a Rover 25, a small car my wife had. It is also hazardous because the anti-lock switches in when not needed, (like braking on a dry road) thus lengthening the braking distance. The rings have in the past not normally been separately available, only the whole joint or bearing, but good news is there are suppliers on the internet and the X-Type rear rings are available. This is a UK site which ships internationally, but I suspect there are USA suppliers for the same thing.