I've heard that 7 series came with LSD's and I wanted to know more information about that and swapping it. did all 7 series come with an lsd or do I need to look for specific ones models? do I have to swap the whole rear axle or just the differential? as a teenage boy with a rwd manual car, I would love to have an LSD especially if I can pull one from a junkyard for cheap but is it feasible?