Hi X5 experts, I bought an X5 2004 sport auto.I have question regarding my x5, where some time the car shaking while accelerating but its not permenant. My questions are, first, if this problem got to do with gear box or transfer box?second, may someone explain what is the fuction of transfer box, is it for 4wd?how much is the cost to change transfer box if i need to change it?any recommendation? Thanks.