This may sound obvious but, have you unbolted the center shaft support?

If so, then the centering sleeve may be "fused" to the rear flex disk from corrosion/grime/age. You might have to pry the shaft away from the disk with a small crow or prybar.

Once you have the disk removed, inspect it carefully for cracks or other damage. They're inexpensive enough so might consider replacement "on principle" (as they state in the manual) if they/it has a lot of miles.

Regards,
R Leo

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Never a dull moment at Berry Hill Farm.