Sounds like your CAD isn't locking.
CAD. Center axle disconnect.
It is vacuum operated. It slides a collar over the short side the short side axle. It acts like hubs.
When you operate the lever that changes the transfer case and vacuum is given or released from the CAD. Then you have 4wd or back to 2wd.
If what you are saying is you pull the lever to engage 4x4 but only your rears spin...this is your issue.
Either a vacuum line is off...the ball rusted or your shift fork broke.
It is very easy to diagnose and repair
Let me know if you think this is your issue and I will help you from there