If you've got a disco 1 back axle (with the disc brakes) then rimmer bros do a spacer set that will take the hubs to PCD 5x120 - which is fairly standard for BMW rims. You'd have to check that the X5 rims are that PCD though, then all you have to do is worry about how far outside the bodyline of your 90 they'll stick out by!
Oh and then you have to find 500 notes for the spacers...![]()