If you're running a viscous LSD a standard driveshaft will not leave enough clearance inside the diff. The spline end is too long, so when your suspension rebounds the driveshaft will smash into the internals of the diff.
I found this out when I tried running a standard driveshaft in my viscous LSD gearbox, we hooked it all up, my mate lifted the tyre to simulate the suspension bouncing and we heard a 'clang'.
Check with Andrew Vibert, I know he should have a viscous LSD drive-shaft because he got my old one!