It sounds like they may have some dry rot... take a pic for us. If you want to go to a tire shop just tell them you're looking to have the wheels/tires checked out to make sure they're okay to be driving on. Driving on tires with really low psi will affect the inside of the tires, I don't think there are any signs to look for on the outside, but when the tire is unmounted there will be a lot of rubber "shavings" in the inside of the tire.

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2003 X5 3.0i Sapphire Black/Black
Sport/Cold Weather/Premium Pkg.