Thanks for all the suggestions. It’s actually the hub side I’m talking about. It didn’t take me but a few tries to get both axles into the differential.

I’ve taken a wire brush, brake cleaner, PB blaster, and a file to the splines but no luck. I can actually see how far the splines reach within the hub from the wear marks on the ID of the hub (the innermost surface of the splines). Seems to me the tolerance is just very tight and it’s possible I’m just not able to get it to go in 100% straight. I considered taking some sand paper to the inner portion of the hub, but decided to just get the right tool and save myself the same headache for the future. I never would have guessed they were like this, though. The oem axles slide into the hubs just like any other car I’ve done this on.

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2006 X5 4.8is