Q: Is it safe to drive the car when there is a differential leak? How much does it cost to repair?
A: a rear differential leak? that is a very rare problem that i do not come across much with A4s so make sure that is the case. i would imagine it would be quite expensive. what i do know is that, if you think all the fluid has leaked out, it is not safe to drive the car. that fluid lubricates the gearset (ring gear and pinion) in the rear diff and without that fluid, it can overheat and cause serious damage.