Seeing as Mitsubishi don't have any Tritons with diff locks - I spoke to an ARB rep today to see when / if they would be making one for the Tritons and they tell me that it's fairly close to happening, though wouldn't get much more specific.
My feeling from the conversation was in the next couple of months.
Obviously they will cost much more than the Mitsu unit, but would still be better than nothing.
Also opens up the idea of having the ARB's front and back..