There may not be a freehub problem at all. Often crap, twigs and string gets caught up by the wheel and wound into the space between the wheel hub and cassette causing excess friction and the symptoms you describe.
Look behind the cassette and if you see a bunch of stuff there, start by cleaning it out. It may be easier if you remove the cassette, but that isn't necessary. If that solves the problem, you don't have to service the freehub --- yet.
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