If it's a 4WD Silverado 1500, the transfer case is generally a rectangular box-like unit located behind the transmission, connected to it via a driveshaft. It'll be fairly large, roughly the size of a small suitcase, and usually has various control linkages and wiring connected to it. Specific appearance details (mounting points, bolt patterns, etc.) will vary slightly depending on the exact 4x4 system (e.g., NV241, NV246) installed in the truck. You'll likely see a shifter linkage or electronic motor connected to it, depending on whether it's a manually shifted or electronically controlled system.
To be sure what you're looking at, finding images online of "2004 Chevy Silverado 1500 4x4 transfer case" alongside photos of your vehicle's undercarriage would be the best way to confirm. Look for the location near the transmission and the multiple driveshaft connections.
