Im not sure if the diff will hold oil, but people are using silly putty to somewhat lock their diffs. Silly putty has a property in which it can be kneaded if moved slowly but wont knead when a quick force hits it (you can smash silly putty with a hammer). Putting silly putty in the diff allows the wheels to spin at different speeds when cornering but when there is a big difference in traction it somewhat locks it.