I bought my car used and only 1 rear tire seems to be almost bald. The other 3 still have about 6-9 months on them. My tire shop tells me that since it's all wheel drive I need to replace all 4. Seems excessive to me. Considering I'm not racing my X5 like an M5, I think only replacing the rear 2 should be fine. What do you guys think?

Also I wanted to switch to the Pirellis - if I only do the 2 rears, think that will matter that much?

Thanks!