I recently installed the Bilstein 5100's on my X. I replaced all four but I drove it once I replaced the rears, before I installed the fronts just to see what the difference was up to that point. WOW. The rear felt great, and the fronts felt like jello. The improvement of the ride once all four were replaced was impressive. Money well spent! $60 for the rear, $65 for the fronts because I ordered them 20% stiffer (glad I did.) Got em from SLR.

Replace the rears, but really think about investing in the front, you'll be glad you did.