Driving along a dirt track this afternoon on my back from a 80 mile round trip and suddenley withoout any warning I lost all drive. I loud grating noise was coming from what appeared to be the rear diff. No drive what so ever I tried to make sure that it had not slipped out of gear both from the gear box and from the transfer box. I tried it in diff lock and managed to get the drive back again to get me home. Once home I checked again in normal drive but still just a grating sound as if the cogs were grating but not catching. Still works when diff lock is engaged.
Any suggestions guys?

sounds like a smashed planet cog or shaft snapped in the diff to me, but it could be a shagged half shaft or a bearing collapse allowing the crown wheel to shift, whatever it is you need to get the rear half shafts pulled out and the oil drained out then pull the diff out and see whats what.