ABS brake

Welcome to 2CarPros.

It's confusing why this would only happen in 4wd because the wheel speed sensor's operation doesn't change. The only thing I can think of would be when the front axles are under a load, the tone ring (exciter ring) is somehow moving and causing an erratic signal from the left front wheel speed sensor.

I attached a pic below that shows an exploded view of the system. If you could, remove the wheel speed sensor and inspect the exciter ring to see if it is corroded, loose, or just dirty.

Let me know what you find.

Joe