Nick, as you're probably know there is a slip joint in the driveshaft. You need 41mm and 46mm open end wrenches to loosen the nut in order to "telescope" the driveshaft when replacing the flex discs. Before you go to the trouble of replacing the flex discs and center support you may want to simply loosen the nut to relieve any accident-related stress that is tending to bow the driveshaft. From my experience the rear flex disc is more problematic so inspect it carefully for cracks.

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Fred Hoelzle