If your diff lock is working you'll know it when you = try to turn sharply on pavement - you'll feel it and hear it with an open = window because the inside rear wheel is wanting to turn the same speed as the = outside, but can't.  If you're sure it is not engaging, you can pull off the = vacuum hoses at the solenoid and see if you have vacuum - plug them back in and = watch the actuator pin to see if it moves.  If you have vacuum but no pid = movement, you know where the problem is.  Cheers.
 
Jim
91 Syncro AW
Alamogordo NM

6. Check the operation of the diff lock.  This = can be tricky, but you should be able to check it by jacking the rear up into = the air and engaging it while the engine is running and in neutral (engine is = required since it uses vacuum to engage).  Have someone pull the knob and = listen for the actuation of the solenoid. (ANYONE ELSE HAVE IDEAS ON CHECKING = THE DIFF LOCK??)