4kq differential locks

[Jack Rich <102466.3676@compuserve.com>]

My differential locks don't anymore.  As my Haynes 4k manual doesn't
cover quattro models, I am at a loss as to where to begin as I don't
know where the system is nor much about how it works.  When I turn the
locks *off*, I hear the vacuum being released, though slightly less
loudly (less vacuum) than when they were working.  Where should I begin
to diagnose the problem?  How do I determine whether it is insufficient
vacuum or frozen differential locking pins?  (Probably the former since
it is suspicious that both center and rear stopped simultaneously.)

Jack Rich
84 4000S Quattro
88 Corvette convertible
90 V8 Quattro