Select a song, and then click down on the iDrive knob again to bring up a small menu. You should see random in there, so just click it.
Don't know if this is what you were asking..

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2011 ///X5 M (On Order)

2009 X5 4.8 M Sport Space Gray/Black Fully Loaded

2007 GMC Yukon XL Denali Loaded and Customised