Where is its location on the 4.9L 6 cylinder engine? Needs replacement.

I can't tell you exactly where it is, but I can give you some helpful advice. Most autoparts stores sell books on vehicles. They're relatively cheap and they come in handy if you plan on owning the vehicle for a long time and doing your own repairs on them. Other than that, the way to find it is to follow your radiator return line from the radiator to the engine. You will see where the hose attaches to the place with your thermostat and the coolant temp. sensor should be right there with it. Should have one or two wires coming from it.

On the passenger side of the motor, below - not underneath - the fuel injectors.

What problem are you having? How did you determine the sensor is faulty?