You'll need someone with a better code reader capable of interfacing with the ABS controller to find the problem. Usually, as most posts have stated, a wheel sensor has failed but which one since there are three or maybe four. Not difficult to replace but you will need a bit of physical flexibility to reach the sensors and associated connectors.
Give Freightliner (assuming the they are the chassis builder) a call so they can direct you to a local chassis service center with the capabilities of testing the system. They might charge you a bit to run the test but will save you lots of time, effort and frustration swapping out good sensors.