I have a 99 ML 320 with about 220K miles and I just measured to the edge of the wheel well for you. 32 3/8" on one side and 32 7/8" on the other. (I was surprised there was a 1/2" difference.)
I replaced the rear shocks two years ago and had quite an ordeal, but finally got them right. Read this thread. Lot's of good info. I posted a summary of my experience on page 6 of the thread.
http://www.peachparts.com/shopforum/showthread.php?t=53524