The four-wheel-drive actuator on a 1998 Chevy K2500 is located on the
transfer case. It's not a single, easily-seen component like a motor, but rather a part *of* the transfer case itself. More specifically, it's integrated into the shift mechanism that controls engagement of the 4WD. You won't find it as a separate easily accessible unit. You'll need to look at the transfer case itself to see the actuator components.
To get a better visual idea, you should consult a repair manual specific to your 1998 Chevy K2500's transfer case type (e.g., NP241, NP261, etc.). The manual will show diagrams and locations of the internal components, which will include the actuator. The location relative to the transfer case will vary slightly depending on the specific transfer case used in your truck.
