Having had an engine refit, upon its return the 4WD would not engage. There was a gearbox "noise" but no light on the dash and the Low box would not engage.
Checked the vacuum system using a syringe on various points- Air Con Diaphragm, 4WD actuator inlet and outlet, vacuum pump union with or without 4WD button on with the engine running. No leaks so checked the Bulb on the dash - the lead to the Transfer Box Switch is live with the ignition on.If the lead is earthed the light on the dash is illuminated. The switch on the transfer box ( near the rear of the box on the top) is an earth when the 4WD is properly engaged.
If you take the 4 12mm bolts off the actuator plate on the nearside, you will see the plunger of the transfer box and the actuator rod, plus fulcrum and you can check if it is engaged correctly and working if someone presses the 4WD switch on the facia with the engine running. No oil should be behind the plate just a few spots from the actuator boss.
Mine was mid way - there was a vacuum when the pipes were removed with the refit and it only partially engaged / disengaged the 4WD upon striking the engine up, hence no light on the dash.