The most common cause would be a bad/cracked tone ring in the rear brakes or a bad wheel speed sensor. This systems uses the wheels speeds to sense if the wheels are losing traction and will even shift into 4x4 for stability. The tone rings can crack and can spin on the rear axles. Take a look at the rear tone rings(wheel or axle needs to be spun to see all the way around), they are mounted on the outside CV joint on the left and right rear axles. The wheel speed sensors are active so they need to be tested or checked with a scanner that shows live data.
let me know what you find
tone ring is not expensive, 25$ and usually 2 hrs labor
wheel speed sensor is about 150$
just make sure the brake fluid is full and no leak from the speed control switch on the master