The oil pressure sending unit on a 2001 Ford Explorer 4.0L V6 is located on the
driver's side (left) of the engine block, near the oil filter. It's a relatively small unit, and you might need to look closely. It's usually a single-wire sensor.
Because engine layouts can vary slightly, it's best to consult a repair manual specific to your vehicle's year and engine for the most precise location. A picture in the manual will be extremely helpful.