rear differential not locking

[Jad Duncan <Jad.Duncan@GAT.COM>]

I have searched the archives for awhile, and even posted a message a couple
of days ago and have heard nothing.  Is the diff lock 1) so reliable that
no one has had a problem with it, 2) so complicated that no one messes with
it, 3) hard to trouble shoot, or 4) everyone is too busy driving in the
snow and holiday shopping to waste time on the list?  I usually get lots of
suggestions and helpful ideas.

Quickly, my problem is that the rear diff does not engage when I press the
button on my '89 90q.  I don't know too much about the system, but after
looking at it, it appears to simply be a vacuum system that moves a lever
in the diff.  Is there a good place to start looking for the problem, pump,
hoses, fuses, (not the differential I hope)?  The system did engage on its
own while going 65mph on the freeway a couple of weeks ago.  Does this give
anyone any clues?

If anyone has any ideas, please let me know.  Thanks.

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