It sounds as if the signal from one sensor at least isn't getting to the ECU. This could be a bad/dirty sensor, or broken wire.

Do you have a staggered wheel/tire set up at all?

An imbalance between the rolling size of the wheels can also cause this.

With regards to testing the sensors, I think you can check the resistance of them, but the definitive way is with an oscilloscope and spin the wheels. And correct me if I'm wrong, but I think the sensor for the rear is on the output of the transmission.