Have you replaced an axle on an E53 X5 yet? THey can be a real PITA and you don't want to do it more than you have to. I just did my driver's front and it was several days of hell getting the old out.

I also made a tool - described in this forum - to pull the new axle into place.

I have bought two E53 axles from CVJ axles in Denver. They sell refurbished OEM axles for the E53 at what I believe is a fair price. There is an add'l core charge of $75 if you don't send your original or OEM axle back, but all-in still a good deal.