Something that i have learned when building diffs, is to lightly sand the flange of the plastic diff case prior to assembly. Prevents leaking where the flange meets the gasket.
Just get some fine to medium grit sand-paper on a flat surface. Take the plastic diff case (empty with no parts or o-rings inside) and turn it flange side down. Lightly moving it across the sand paper back and forth while turning it slightly. Only remove enough material so that it removes the "shine" from the mold and is now flat. Clean the diff case and be sure there is no debris in the threaded holes.
This should help with leaks at the flange.
If leaking at the o-rings, use a little more grease on assembly and give the o-rings time to seat or swell after break-in. This shouldn't take more than a few runs.