That car had about 4 months of racing Mod and in that time I never had any problem with the diffs - they are the most reliable part of the car!
Are you sure sure that they're still loosening off?
As the diff lube starts to come off, the diff will have a looser feel to it, but the screw should still be set tight. If any diff was to loosen, it should be the rear diff, as it has the most load on it.
The diffs on that car often felt very free and loose, but they didn't slip on the track.
As for the one-way, the primary advantage is better turn-in when compared to a diff, however you do lose the ability to use brakes - which can make it a bit of a problem on tracks with hairpins
There is no centre one-way option, and the normal one-way will cost you around AU$100 - ask HeavyD for a price.