91 S10 blazer front differential doesnt always engage

My front diffenential shift fork doesn't seem to be engaging all the time. I have the truck on jackstands and have checked the vacuum selonoid and vacuum system. I am getting 19 inches of vacuum to the vacuum actuator under the battery box. It is retracting like it is supposed to. I disconnected the front differntial cable from the vacuum actuator and tried to engage the front wheel differntial using pliers to pull the cable. (engine running and in 4 wheel high) Sometimes the differential will engage and most of the time it doesn't. Sounds like (and feels like)it is close to engaging but you really have to push the cable in and out with the pliers serveral times to make it engage.

Is there any adjustment to the cable to make it more positively engage? Or does it sound like the front differntial is shot?
thanks,