thebax2006
One of the wheel speed sensors is bad. You need an ABS capable scan tool to read the ABS codes to determine which wheel speed sensor it is.
If the sensor bolts on to the knuckle make sure to clean the rust off of the knuckle before installing the new sensor.
CactiJoe
Sometimes it isn't receiving a signal from one of the sensors (ABS sensor) on one of the wheels. Sensor could be bad or the wiring to the sensor further away from the sensor. Salt water can sneak into the sensor plug connection and corrode the inside of the plug or wire connector.
Anonymous
Check for trouble codes with a dealer level scanner. Likely a bad wheel sensor.