I don't often drive in the snow, but because I'll have my transaxle in pieces soon I'm thinking of adding one of these-

http://www.quaife.co.uk/What-is-a-Quaife-ATB-differential
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10101311 (20GB- backup empeg)
10101466 (2x60GB, Eutronix/GreenLights Blue) (Stolen!)