if you leave that driveshaft nut loose the splines will wear resulting in purchase of your future new driveshaft. It allows the shaft to be shortened for replacement of flex discs and its removal (DB didn't see the need to remove the differential just to access the driveshaft). After new discs, center bearing, etc. you rotate shaft a few times to allow final length set, then tightening the large nut is the last thing you do. Nothing moves enough to need a domestic type sliding spline. I just did this on newly aquired 280SL, had a whine/noise so bad I thought the differential wouldn't make it the 1600 miles home. Fluid checked fine, but found the worst flex disc I ever saw (almost metal to metal on the yoke mounts). Since it was only a flex disc, opened window, didn't hear it as bad, and motored home. Replacement of both discs and tightening the center nut (found loose), plus rear subframe mounts eliminated almost all noise and play in the drivetrain.

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Hanno
'79 6.9 Sold (after 27 years)
'83 280SL, 5 spd.
'94 E320 Sdn. 5 spd conversion
'02 E320 Sdn.(on loan to mom!)
'87 300E (5 spd. conversion) Sold
'05 E500 Wagon