My dealer refused to change the diff or trans fluid. Service tech drove the car with me in it and said both were performing fine (they are). Free service means very little because there is very little specified servicing.
With most dealers, you will be on your own. BMW has clamped down on unnecessary work. They can get away with wiper blades, which is why you will see a note along the lines of "wipers streaking, blades replaced" at every oil change even though you never complained about this.
I bought brake fluid, power steering fluid, diff fluid, and trans fluid and will be doing my own "pre-lifetime" change. According to BMW, diff and trans are "lifetime" fluids.