The linear actuator motor is located on the passenger side front axle tube. You will see the wire harness plugged into the axle housing. These were a bad idea for GM. It probably needs replaced.
There is a cable operated conversion kit that can be installed in place of the actuator. Let me find it. Here it is at LMC Truck Parts. It's called 4X4 Posi-Lock and it's about $300.http://www.lmctruck.com/ICATALOG/CD/full.aspx?Page=130
Here's the factory replacement. It's about $110 to $170http://www.lmctruck.com/ICATALOG/CD/full.aspx?Page=128