Exactly.
Difference in size will render the AWD system to think that the front tires are spinning, so will indefinitively try to compensate.
Do a
on the WEB, you should find article that talk about just that.
I was a mechanic, but for not having experimented it myself, I can't tell you what kind of damages, at what speed, etc.
Not a good way to transform your X5 as a rear wheel drive...
