ABS light is on and traction control is off. Where to start?

I suppose it's possible a wheel speed sensor fell out and is banging around, but the bolt would have to work loose, then the sensor, which is normally rusted in place, would have to back out. The signal would drop out long before the sensor could come all the way out, so that's not likely the cause. It's more likely the wire harness fell of its attaching clip and got hit enough times by the tire to cut it.

If it's an electrical problem like a cut wire, the computer will detect that during its six second self test when you turn on the ignition switch. The yellow ABS light might go off for a fraction of a second but it will turn right back on. If everything is okay electrically but the sensor is spaced out too far, has rust or metal filings on the end by the magnet, or the tone ring is rusted and cracked off, that won't be detected until the truck starts moving and signals show up from the other wheel speed sensors. That's when the light will turn on.

A mechanic with a scanner that can access the ABS Computer will be able to read the stored diagnostic fault code(s). That will direct you to the circuit that needs further diagnosis.