Often the old vacuum operated 4WD stuff seizes up due to lack of use. If it has an external rod from the vacuum pot to the diff see if you can get in and move it with your hands /WD40 everything.
Most likely seized slide pins.
See if you can undo them. It's the two 12 or 14mm bolts top and bottom of the caliper.
Be carefull, if they are completely seized you might snap them. Then it's time to find a second hand caliper.