4KQ Rear Diff. Problem

["Guillermo S. Christensen" <gchriste@CapAccess.org>]

While driving my 4000Q on a twisty unplowed back road (10 inches of snow
on the ground, best guess) I locked both the center and rear differentials
to get extra traction (with the diff's open I could feel the tires
slipping from side to side as they hit better/worse traction).  I've
locked the center diff. a couple of times but never the rear, since
snow/rain/ice around here is not usually heavy enough.  After I drove onto
plowed main roads I tried to unlock the diffs, however, only the center
unit unlocked (the indicator light went out, and I could feel the
difference when turning).  The rear unit remained locked (with the
indicator light on) which I could confirm when turning (the inside wheel
hopping along).  The center unit can be locked and unlocked at will.  I
pulled the center console panel off and checked to see if the vacuum
switch is working:  it makes the right sounds (hissing when the switch is
moved and the vacuum passes from one hose to another).  Is there any other
troublshooting method that springs to mind to anyone on the list?  I hope
to jack the car up tommorrow to look at the differential itself, and if I
don't see anything to take it in to the mechanic on Monday.  TIA.

Guillermo S. Christensen
Mclean, Virginia
'93 BMW 325is
'84 Audi 4000S Quattro