OK ... clutch centre is one possibility, not far behind that is the mainshaft splines where the mainshaft from the gearbox goes into the transfer box - there's plenty of info on here on the fault if you have a search, but basically the splines strip and you lose drive - its not hard to check and is a possibility on that age of box, on new R380 boxes the fault has been cured. In fact unless you are 100% sure it is the clutch, its easiest to check that first as you only have to take off the PTO plate on the back of the t/box.