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4kqs question(s) - longish

Hi all,
    I've been lurking on this list for over a year now.  I recently
began having some trouble with the above car and hoped someone here
could help me out.
    The car in question is a 1985 4000 S Quattro which I have been
driving for about 3 weeks.  This car has been fine until tonight, while
driving home, I suddenly heard a loud clunk and the car lost all of its
power.  The engine still continued to run fine, but the car acted as if
the transmission had dropped out (figuratively) - the engine would rev
up like normal but no power was being transmitted to the wheels.  I
coasted the car over to the side and road and attempted to shift it into
several different gears.  When I shifted into 5th and let out the
clutch, I could hear a whine coming from the transmission which lead me
to believe the throw out bearing was spinning but still no action was
occurring at the wheels.  I tried several other things and just before I
was about to give up, I decided to lock the center differential to see
if anything would happen.  Lo and behold the differential locked, the
wheels engaged and I was back in business.  I drove the car the rest of
the way home this way and pulled it into my garage.  Once in the air, I
pulled the pin in the end of the transmission which locks the center
differential to the "out" (unlocked) position.  I lowered the car to the
ground, and it seemed that it was then mysteriously healed.  I decided
to go for a test drive and pulled out of my driveway.  It backed out
fine, but as soon as I started to accelerate, I again heard an audible
thud and the car was back to its old antics.  I locked the center
differential again and was then able to drive it back into the garage.
    I did notice when I first purchased the car, that the all the
differentials would lock and unlock except for the center differential.
It would lock, but remain locked and had to be unlocked manually by
getting underneath the car and pulling on the lock.  My question is, is
it possible that since there is no (apparent) vacuum to the center
differential servo to unlock it, the pin in the end of the transmission
works its way out until it reaches a "dead zone" where the center
differential is not being engaged?  This would make sense to me because
if I understand Bentley correctly, no power would be supplied to the
rear differential either.  If this is the case, is there a typical place
where the vacuum hoses going to the the center differential servo wear
and leak?  I would greatly appreciate any comments/thoughts.

Thanks, Zain Horning
Mail me at mailto:zahornin@liberty.edu