On a 2003 Chevrolet Silverado/GMC Sierra 1500 with the 5.3L Z66 engine (Vortec 5300), the crankshaft position sensor (CKP sensor) is typically located at the
rear of the engine block, near the transmission bellhousing. It's usually mounted on the engine block itself, and you'll need to get under the truck to access it.
Important Note: The exact location can vary slightly depending on specific vehicle configuration, so while this is a common location, it's always best to consult a repair manual specific to your year, make, and model. A repair manual will have diagrams and precise locations.