I don't think it's the Bowden cable, because if it was all the gears would shift late. I would bet it's internal wear on the k2 clutch. These boxes are pretty easy to rebuild if that's the case. I did one on early 90s SL in about an hour on a bench.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k