80 series centre diff lock not engaging

Bucky
I would be looking closely at the wiring loom condition and for any damage or having been chewed by rats etc. I f sitting unattended for a period, a rat attack is highly likely.
My SIL has a V8 Commodore next to his house. In the engine bay below radiator there is 120 mm of a thick wiring loom completely GONE. About 12 wires cut cleaner than I could with sharp side cutters. No joke!

I've been travelling with 80's that have had this problem a couple of times over the years and both times it's been the actual motor on the actuator at the transfer case, water ingress into the little electric motor and little gear that does the work. The first time on a relatively new 80 it was replaced under warranty the second time was out in the desert after lots of water enroute and the bloke actually had a spare unit that he had picked up from the wreckers aand had carried for several years.
The original in both cases was beyond repair.

Good luck.. this is probably not going to be cheap to repair.. relevant video..

https://www.youtube.com/watch?v=LCZSMHCdji8

Most of these systems have a time out function, so when the action is, or is not achieved, there is a halt to the system. If it can achieve the action, there is a feedback circuit which closes via contact or switch designed into the motor unit housing to tell the Controller ECU it has arrived in position. If not, it flashes and times out. The ECU controller simply reverses the motor to take out the lock and again a feedback switch operated the other way to close a contact to say it has been returned to OPEN. Then the controller goes into idle mode as is normal.

The mention of a vacuum operated unit just increases the complexity with NO motor to make the change, but a vacuum chamber controlled by electric vacuum valves to operate the changer. Therefore, two conceptual systems to deal with if problems arise.