I would start by spraying the shaft where it enters the side of the tractor.
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I know it's couple of years old post, but in case there are others with the same issue:
Be careful what you spray in there!
There is a tiny O-ring and that's all keeping HST oil from leaking out.
I would just use the same HST fluid first and try to wiggle.
The PTO shift lever is on the right side with the end barely visible in my photo, but it has an O-ring seal just like the bigger clutch lock lever on the left side that's partially pulled out.
All the PTO gear shift lever does is control oil flow to the PTO clutch, unlike the clutch lock lever.