On my 9-3, that's supposed to be done by the traction control system using the service brakes on the front wheels. A few of months ago, I got the chance to test it for real in the saturated parking field at a kit-car show. I can report that it doesn't work. After loads of goes with the traction control on, I was getting nowhere. I tried gentle rocking, all out thrashing and everything in between, but all I got was amused looks from onlookers. The spinning wheel was being braked all right, but the system kept backing off the throttle until the engine bogged down. When I finally turned off the traction control, I could make very slow progress towards firmer ground. I reckon a proper LSD would have worked wonders.
I love all this "it's not a real XXXX unless it was made during some random period in the companies history" nonsense. It brightens up the relevant alt.autos newsgroups enormously :o)
Personally, I'll always maintain that the last true SAAB was the 18...
Cheers,
Colin.