4000 Q diff. locking problems

[Rosso75@aol.com]

   I'm new on this particular digest and new to Audi as well (though I've been
into VW's for years), so if this is something that any moron should know, or
it's been covered to excess, my appologies in advance.
   I just got an '86 4000 Q with 169k on it.  The original owner had done all
the service, and the car runs great, but it's got a couple problems.  
  First, the rear end is howling.  The seal on the drivers side is leaking
which I've heard is common and I'm in the process of replacing it.  I thought
it might be just low fluid, due to the leak, so I drained it and re-filled it
and it still made noise.  I'm now in the middle of replacing the carrier
bearings.  The wear patterns on the ring gear and the spider gears look good
so my first thought was that it's the bearings, but the bearings don't really
look bad.  They've got obvious heat damage (discoloration) from the low fluid,
but no pitting.  I'm going to replace them and put it back together and see
what happens, but I was wondering if anyone had any similar experiences.
  Second, neither the center or the rear diff. will lock.  I pulled the center
console apart and came to the conclusion that the system is vacuum operated.
I traced the white hose from the center two positions on the switch through
the firewall and into the engine compartment.  I checked to make sure the line
is clear and it is.  However, I get no vacuum pressure at the switch when the
car is running.  I assume I should.  My guess is the switch directs vacuum
pressure to different lines that actuate levers that engage and disengage the
locking diff., and that being the case I further assume I should have a
constant supply of vacuum at the switch.  Any thoughts???
  Finally, the car in question is white and has red leather interior.  I've
been told this is a pretty rare combination for some reason.  Yes?  No?
Thanks all......
Bryan Carter