4000 cs differential locks malfuctions

[JONATHAN WOLOVICH]

Hi every one.
 
The system in my car is not working with the knob commands.?
I have been working in this car 3 years with all source of problems.
Now, I need help again.
 
I went  under the car and shifted the rear differential manually to unlock position.
It worked in front wheel drive for a few blocks and it tried to go in lock positing by it self.but  it wont engage properly,  so it makes a gear spinning noise.
 
Where do I start checking the vac system?it seems very complicated to find where the leak might be cause nothing in the system really works.
I know there is a engine vac. pressure switch under the battery which, it was broken.
I fixed it but not sure if really works either.
 
Does any body know where, do I find such part and or how it is called part # it it very similar to the one at the rear end.(plastic to houses)
Is it there and electrical devise which can be use instead of the vac shifter?
 
Thanks
 
All recommendation will be appreciated.
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