Generally, it's either:
* In the fuel tank: This is common in many modern vehicles, meaning you can't access it easily without dropping the tank.
* On the frame rail: This is another common location, often near the fuel pump. You may be able to access it from underneath the truck.
Without knowing the specific engine (e.g., 4.3L V6, 4.8L V8, 5.3L V8, 6.0L V8, 6.2L V8, etc.), giving a precise location is impossible. Check your owner's manual first.