The most likely cause of a sudden and unexplained stiffness in the forks is that they have become misaligned for some reason. If they aren't parallel in the yokes, there will be additional resistance making the forks sticky and unresponsive and will quite often pack down after a series of bumps.

They're pretty easy to realign. Here's a simple to follow video that explain what you've got to do and why you need to do it. It's not an XT, but the principle is exactly the same.



There are other reasons for stiff forks, but it's always best to start with the simple/quick things first.

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