shadrack

Hi, ive a 2007 ml63, anyway when cornering slow its clicking like a cv joint sort of noise and getting steadily worse over last few months, both directions but only when steering, i dont reckon its a cv joint though as if you are reversing (more prominent) or forward on full lock on a loose surface then the iner wheel is almost skiding / locking up on the surface, so im thinking either front diff or transfer box?

1) anyone got heard of this issue
2) what fluid go's in the transfer box as i could try a change, i found a similar issue listed for a 4x4 car version merc and i said mercedes fix was a oil change to transfer box, thanks