Make sure there are no residual gaskets pieces or sealer left on both sealing surfaces, on the rear end housing and the mating flange surface on the third member
Good luck, to bad about the leaky one.
Let us know what you think was causing it to leak after you get it apart
BTW, I have had used 8 3/4 seep out of the threads on the lower studs due to the soft copper washers being removed and not reused A good silicone sealer on those threads are your friend