I've spent a bit of time looking into it a little more.
A spare parts guy at MB had a good point. He suggested that if the gear ratio was changed, the wheel/engine rev ratio would also change- take care the computer doesn't perceive this as a major problem and put the vehicle into limp mode.
Chrysler service just about had heart failure when I suggested a gear swap - does anyone know if Chrysler are capable of reprogramming the computer if the diff ratio is changed.
I am going to try the MB car club next to see if anyone knows diffs but I don't hold much hope - they tend to breathe different air.