I agree, since it has been a problem from day one, I'd start with the thermostat and check it for two things, one it is not installed backwards as that will cause your engine to heat up a lot more before it actually opens to allow flow through your radiator, and two, check the thermostat for proper operation at the correct temp.
Take your thermostat and push it open and stick a long string between the valve and the body and let it close on the string. Now take a pan of water and heat it up with a cooking thermometer in it and hang your thermostat in the water by the string and see exactly at what temp it opens and falls off the string.
Now you REALLY know at what temp it is opening and allowing coolant to flow to the radiator. It's a starting point to know what is truly going on in your cooling system, and a process of elimination without starting to replace parts that may still be hard to come by.
If you find that your thermostat isn't opening at the right temp, either too hot or too cold, you need to replace it. Surprisingly a thermostat that opens at a temp that's too cold, can cause an engine to heat up. The reason being, the thermostat also regulates how fast coolant flows through the radiator, and if it flows too fast, it doesn't spend enough time in the radiator to dissipate the heat, and the coolant carries that heat back into the engine and it starts getting hotter and hotter.
If your thermostat isn't installed correctly or not working properly, the radiator size and design means little, and after all the diesel engines they have built and mated to the radiator, they know what works and what doesn't, no matter what you have added on to the vehicle as far as weight or driving habits.
I want to say more, but I'm working on my posts lengths....how am I doing CJ2Rox...lol