1996 GMC Jimmy Four Wheel drive won't engage

I have a 1996 GMC Jimmy with 210000 miles that will not engage it's four wheel drive. I have checked the vacuum and it is showing 18 in. The vacuum actuator appears to be functional and pulls the line to engage the front differential about a half inch. The transfer case and front drive shaft engage and function fine, but the differential will not engage. It was replaced with a lower mileage unit before I bought the car. How far should the actuator be moving be moving the push-pull cable to engage the differential? Where can I find a diagram for this system?