I just replaced the rear tires on my 01 X5 after doing some research that suggested that it is ok to have slightly different sized tires (due to older tires being up front). This is one article I read:
http://www.4x4abc.com/4WD101/diffautoAWD_AWD.html
Excerpt from that article.
"There are some AWD vehicles where it does not matter what you do. Replace one tire, replace 2 - it does not matter. Those AWD vehicles are true AWD, in the sense that they are full time 4WD or permanent 4WD powering all 4 wheels with about equal force. Most Subaru (except Impreza), BMW X5, Audi, some Toyota, Mercedes, etc. are in that group of true AWD. These AWD vehicles have a differential between the two permanently powered drive shafts. They are the ones that will survive a replacement tire that is slightly larger or slightly smaller (like when you have to buy a used (larger) tire in Mexico because nothing else is available)."
But searching around this site has made me question that decision. Any input from you guys?