2008 Inifiniti FX35 Tire Problems?

I have a 2008 Infiniti FX35 with 20' original tires. i have 20K miles on it and recently i have notice that all four tires are cracking. It looks like the rubber is cracking from old age. I took it back to the dealer and the Goodyear dealer but no one wants to help. I don't want to pay for brand new tires since i only have 7 months left on the lease and Goodyear gives a 50K warranty anyway. can anyone help?