you can leave the fluids in it, just when you pull the axles out your gonna need some plastic bags to keep the fluids from coming out, but you can drain it if you want to.
They dont take that at parts stores, just oil.. BUT you could get away with putting it in the tank..

they wont know, or prob care.
The axles arent hard, if you got a breaker bar..its easy, if you get lucky it wont make you curse too much. If your rims allow you to, you can loosen the axle nut before you jack it up..and save time.