The viscous unit is a sort of LSD, after all it limits slip, right?
The very early RRC had the LT95 with LSD in the centre diff and also IIRC had difflock full-on (if only they had done that with the B-W box :roll: ) so you get the best of both worlds. I could be worng here but I think it was merely the case that the LSD got deleted, not sure why though :? cost or reliability.
An LSD will not drive quite the same as a viscous though, you may feel it 'unlocking' and it will be more prone to oversteer.
Oversteer, what am I saying, get it chucked in :D