Well the first thing you will need to do is check for codes. You will need a scanner to do that.
I can only tell you the most common thing I have seen, which could be your problem.
The rear hubs from what I have seen build up rust due to the water channel in the knuckle getting clogged. So when this happens, the rust build up on the hub now effects the distance of the sensor and most cases starts to rub on the sensor which shorts out the sensor. Then the next phase after that is the sensor shorts out the ABS control unit.
So you will need to check the codes. If you have any rear ABS sensor or control unit code, check the rear sensors. You will see one of the corners have a rub mark. Check both! The hubs may need to be replaced and the knuckle may need the water channel grind down and opened up so it doesn't happen again.
If you have any more questions on this, please feel free to ask.
I will be out tonight and won't be home till late, so I may not be able to respond if you have any more questions till some time tomorrow.