You need to check the rear toe and camber and set it to within spec. You should also check those pesky rear subframe bushes to make sure the carrier is not floating about. Powerflex bushes for these are the way to go.
Powerflex
Can be found discounted on eBay.
You will almost always get heavy rear tyre wear though on the X5 due to the geometry but keep the pressures within spec and with the Powerflex bushes done and an alignment, I have now done 22000m on a set of tyres that are now about 3/4 worn.