You can get a 2.5mm allen wrench in one side and see if tightening it up helps. There's a bolt that goes through the diff and holds the balls, spring, washers, and d-rings in place. If that gets too loose, the diff will slip.
Its going to be inside one of the out drives of the diff. I couldn't tell you which side though, its been years since I messed with one of these. Just pop off the top ball cups and check each side, tighten it until it drives like you want it to. If it won't tighten enough, its time for a rebuild.