Your drive shaft is spinning but the wheel on your Chevy 4x4 wont turn when 4x4 is engaged?
If it has lock out hubs, Then you have a bad lock out on one
side are the other.
If it does not, Then the Actuator on the front housing is not
ingaugeing. Need to check that with a test light. If it has power
going to it , Then replace 4x4 Actuator.
If no power then check 4x4 fuse in fuse box.