Filler plug for transfer case??

Thanks Leo109, thats very helpful info.

Further to what Ian said, you really should have the centre diff locked on loose surfaces and most definitely where you might get wheel spin.

Spinning wheels with the centre diff un-locked can destroy the diff - they are only intended to compensate for the small differential action that occurs on corners onroad. They are not very robust and the thrust washers and pins flog out when the wheels spin too much, which leads to pinion failure.