First order of business is stop working it until this is fixed. This isn't your Grandpaw's gas powered engine that can survive some overheating without permeant damage. If a diesel engine isn't shutdown at first sign of overheating it can easily crack the head,melt piston, blow head gasket, destroy cylinder liner and other things you don't want to hear. NOW , are you going by a light,guage reading,steam escaping or other indicator? If you are lucky it's only a bad sensor and the way to know is with an accurate laser thermometer or temporarily installing an accurate sensor and gauge. As mentioned above , dirty radiator fins is most common cause of overheating, have you checked that? Auto parts stores sell a chemical you put in coolant to detect exhaust from cracked head,blown gasket or other failure allowing exhaust into coolant.
That should keep you busy for a while and tell you more than us wagging at it. After you finish come back to report what you found.