If you have ball diffs it's even easier, just take the 3mm balls out of the diff gear and tighten the diff up until it's solid, then if you don't like it or want to set it up as a gripper just drop the balls back in

...BTW you can't do this with a geared diff

Another thing, if you intend using rubber tyres + solid diffs, be prepared for outrageous wear on the outdrives [X(]

Kev