CobraK and Snakester, I've got the same thing happening. I've got a little over 4K miles on my car and the leak has been getting worse. I wiped the frame down to see where it's coming from. The "spline" bolts just next to the diff (left side only) are really wet, but the CV boot is dry. There is nothing coming from the speed sensor, so that O ring is Ok. I found this diagram of a BMW rear axle parts diagram on the web:
http://www.realoem.com/bmw/showparts...62&hg=33&fg=10
I don't know if it's the correct diagram, but it is for an E36 Bimmer.
I went to a friend's place Sunday and the car left a little 50 cent piece size drip of purple on the ground. Ok, so now I know it's Royal Purple differential lube.
I'll keep watching this thread. If I learn anything more, I'll let you know.
Hank Steckel
BDR #258