I don't have a direct answer to your questions, but I will give you some input anyways.
The ABS sensor is a proximity senor. Every cog or "tooth" on the toe ring sends back a pulse to the computer. It is the stream of pulses that the ABS system measures to determine how the braking ABS system is to react, or how the traction control engages (if you have that option).
If the toe ring looks intact with no broken "teeth", or plugged up with mud or something, then I would think it is the sensor.
The service manual simply states that the sensors are bolted to the front wheel knuckle and rear wheel spindle.
You probally should spray liquid wrench around the sensors before attempting to remove them.
