In order of likelihood: (IMHO of course. YMMV)
ABS Controller: Confirmed by shorter and shorter time to make the light come on. Eventually it will come on right away. Known fault with controller circuit. Fix: reman unit (you can take it off, send it out, and it won't affect your brakes since your abs is already out) another name besides BBA is Module Master. You don't need to clear the code when it's replaced, the light will just go out.
Wheel Speed Sensor: they do go bad sometimes. If you can, get yours tested before you replace because they're not cheap for what they are (basically a magnet on a wire). If it's the right rear- the speedometer will usually be non-functional.
Voltage: Usually, you will get other messages like Trans Prog when this is the issue.
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