The most common ABS problem with Workhorse units is dirty wheel sensors.
Slide under the rig, on front tires behind the axle there is a device with a wire going to it, ti's round (Save for the cap where the wire goes in) cylindrical in shape, about the size of a roll of Life Saver's candy, it's shoved into what appears to be a brass tube... Twist gently and pull it comes out.
Clean it well and shove it back in (Brake parts cleaner should work, and a rag)
Then do the other front wheel
Then do the back's on the rear's the sensor is in FRONT of the axle.
NOTE: That is the most common issue.. NOT the only possible cause.
Home is where I park it!