X6 rears have a different center bore than the X5, so you need to have the center of the rear rims machined to be the correct size - fronts should fit fine.

Fronts on both X5 and X6 are 74mm, rears on X5 are 74mm, rears on X6 are 72.5mm, a good wheel refurbishing place or machine shop can easily bore the X6 rears up to 74mm.

The other option is to run some spacers that will adapt down to 72.5 - they run about $120 for a pair for the rears (and helps the looks a bit also) Home Page > Search > es1899557 es1905746 - ECS Tuning Inc