Hello JDD,
My X5 is basially the same as yours. Auto/steptronic, 3.0d turbo, 18" wheels, all stock equipment.
I haven't abused it in the almost 3 years that I've had it and I doubt very much that the somewhat elderly gentleman, from whom I bought it, had either the time or inclination the put it through those paces.
I am inclined to agree with you on the idea that the material was in someway not properly hardened. It's my only logical guess
I have since ordered and received two new pieces and shall proceed to rebuild the differential ASAP.
Thanks for the questions and ideas. I love the learning process!