Replacing the gear is an easy fix. 'flipping,' e.g, rotating the gear to wear it in a different place, likely not enough. I rotated mine and a month or so later pulled the transfer case and replaced the gear. No problems since ......
These vehicles are 10+ years old. No way I put $4k into replacing what almost certainly isn't needed. Get it properly diagnosed and you'll likely find all you need is a new gear.

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2006 X5 3.0 6-spd w/Evo UUC ssk
sport/premium pkgs
born Valentine's Day, 2006.