It's a pretty simple job. Most people will suggest GNK or OE axles. I was in a pinch and needed my X the next day when I found a split boot, so I bought a reman axle from NAPA and installed it. I expected to replace it once it failed with a quality piece, but it is still going strong 10k miles later. I did have to go through a couple at the store to find one that wasn't either defective or damaged.

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03 3.0i mt
89 325is