The 4WD vacuum actuator on a 1991 Chevy S10 is located on the
transfer case. More specifically, it's usually on the
passenger side (right side) of the transfer case, near the front. However, the exact position might vary slightly depending on the specific transfer case model. You'll likely see a small, usually cylindrical, device connected to vacuum lines.
To be absolutely certain, you should consult a repair manual specific to your 1991 Chevy S10's year, model, and transfer case type. A visual inspection under your truck, with reference to a diagram, would be the most reliable way to locate it.
