Dump the oil. take the rectangular inspection plate off and have a good look around. Centre diffs take a hammering when people go off roading and just spin the wheels up whilst in diff lock. I see people do it all the time - if we get warranty returns on lt230s its because of this. Take a look inside yours and atleast you'll have a better idea of what happening inside.
Good luck
Shabs @ Syncro Gearboxes
http://www.syncrogearboxes.com