Diff grease
 

Is it possible to add differential grease inside the WK axles and not have it ooze out? I'm trying to go for a posi effect rather than a full lock.

If you want a posi diff efect try cleaning out the diff and the putting a small amount of silly putty in it. I've heard it works well as a kind posi diff.

Yup silly putty would do it. It's like diff grease, silly putty is a non-newtonian fluid. It will flow when the stress on your drive line is low allowing your differential to stay open, but as soon as you increase the stress on you drive line it will harden and lock your differential.