HELP!!! Rear Diff Problem?

[Jason Stone]

I've tried to include as much info as possible - I
will answer any questions to help me know where to
first look...

Help!

Problem: 
========
Loud clicking noise coming from rear of car - pretty
sure it's the rear diff - it does go away but left
turns always bring it back.  Something loose in the
diff?  The other thing - the rear diff is now locked
and the ABS is off - the switch to unlock the centre
diff does not unlock the rear diff - the switch is
working I think cos the light on it goes out above
40km/h (roughly).  However the diff remains locked and
the ABS off even up to 140km/h (I was hoping to get
some speed to unlock it)

Observations: 
=============
When I got home the rear diff is leaking fluid/oil :(

Cause: 
======
Had to reverse down a snow covered road -
unfortunately not enough snow and got caught in
someones tracks and the car slid sideways and into the
ditch on the side of the road - more than likely the
centre of the car grounded out. The two left wheels on
ice and the two right wheels in a deep ditch full of
snow - enough that I was just compressing it and could
not move it.

Finally after quite a few attempts at driving out I
dug tons of snow out from under the wheels and got a
Jeep to tow me out.  When I get out I hear a loud
clicking from the rear - but it does goes away - but
comes back when I turn left.  When I turn left I feel
a weird movement from the rear - something to do with
the diff? Feels nasty (for lack of a better
description.)

Questions:
==========
* What could it be? 
* I want to sell the car anyway - change the broken
gasket - new oil and then hope the noise goes away?
Use Redline oil MT-90 ??
* Could it perhaps be something like a driveshaft? 
Stuffed CV? Perhaps the oil leaking is a red herring -
just a broken gasket maybe?

Jase
'91 Audi S2
'97 GSXR600

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