If the front half of the driveshaft has a different amount of lash than the rear half, then obviously there is some problem in the center joint of the driveshaft. Can you get a helper to hold the front or rear half still (don't let it rotate) while you try to twist the other half? The driveshaft should rotate as 1 unit, not half rotating a certain amount and the other half doing it's own thing.

Gilly