The ABS compares the readings it is getting from the 3 sensors. If all readings are the same or very close, the light is OFF. If you jack the car up and run the rear wheels and the light is off then the system is not seeing the speed of the rear tires, so it thinks all is good (Light off). If you then jack up the front and spin a front wheel then the front and rear don't match so the light comes on.
I say that the front sensors are working and the rear is not working. Replace the rear sensor.

Paul

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84 500 SEL (307,xxx miles)