In an off road, I would recommend a 4 plate self-locking diff. They're basicaly limited slip diffs. When torque is applied to the diff, it locks both axles together. When torque is released, it unlocks allowing you to turn and remain stable giving you the best of both worlds. If you want the best of the best, an Oddified Mag diff from Ian Oddie would be the ticket. It's a completely different design than any other diff out there and works in all conditions in all cars. It works by using Eddy current generated by 2 aluminum plates fitted with magnets to lock up the axles when one is diffing out. It's basicaly an electric motor in reverse used for braking. I've been using one in all my cars for the last 2 seasons and won't use anything else.
http://www.rctek.com/phpBB/viewtopic.php?p=36948#36948
Here is a thread I started on RcTek that Mr. Oddie gave a full rundown of the different types of diffs and how they work, and which ones work in different conditions.