another 4x4 question

I was looking at the jimmy today and noticed when you have the truck in neutral and push the 2wd to 4hi or 4lo button you can here the encoder maybe underneath the truck trying to engauge but it doesn't go into 4x4.It makes like a cylinder stroking sound but thats it no 4x4.Should I pull the wiring harness off the transfer case to see if there is power there at the motor?Not sure new to all this.Also can the encoder motor be rebuilt if thats what it is?
Thanks for all your help guys.
Trev

Looking at some stuff today.I read a post about a battery leaking acid, so I took the battery out and the Vac. actuator diaphram was totally destroyed,looks eaten by acid and the cable to the trasfer case was rusted out,so thats what my problem was.Hope I can help someone else out with the same problem.
Thanks for all your help
Trev