The rear flex disks generally last two or three times the front disk, so don't spend the money unless you know it needs to be done. It doesn't wear because there isn't as much lateral movement at the differential. Most of the vibrations and movement and taken up at the front disk.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
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2003 Gray Mercedes ML350 - 122k