The fastest way to find this is with a scanner that displays live sensor data. It will show all of the wheel speeds and if one signal is dropping out. The front-wheel-drive cars have a huge problem with the sensors that are built into the front wheel bearings. I haven't heard of that problem with the trucks, but the scanner should provide a clue as to what's happening.
Unlike the cars, your sensor is exposed and metal particles could build up on the tip of it. That can cause a weak signal.