Axle boot spraying oil

The axle boot (on the wheel end) on my rear driver's side axle is cracked, and is leaking oil - it appears to have started spraying oil all around while the car is in motion... this appears to be thin oil and not grease - where would that be coming from inside that boot?? I could understand if it was the boot at the differential end of the axle, and an oil seal was blown - but that end doesn't look bad... I guess I don't know enough about how this all fits together down there.