and I'm pretty sure the ABS fuse is under the hood. I think I remember seeing it there.

If it were break booster, I imagine the vibration would get worse if you were to press harder on the brake pedal after you stop. If you are stopped, and the vibration starts, push way down on the brake pedal and see if the vibration gets worse/better/stops/kills the truck. Something will change if it's a vacuum issue with the breaks.