Bill,
Check out these torque biasing diffs. Wavetrac makes them for BMW also and they are about half of the price you were looking at. No clutches all 9310 gearing and stunningly smooth through the corners. I have one for the 8.8" Ford and there is absolutely no POSI carrier that even comes close.
Click here =>
Wavetrac
Almost forgot they come with a transferable lifetime warranty.
Ed