I don't have much diesel experience, but I've chased overheating problems on gassers so I'll take a shot.

First of all, are you sure it's really overheating? If it were me, I'd check the hoses and radiator with an IR thermometer just to rule out a bad gauge or sensor.

As others said, do the obvious easy things first: fan belt tension, coolant level, good t-stat and radiator cap.

If that all seem OK, I'd find a way to rig the fan clutch so it's always locked up. Use rope, wire, nuts & bolts, something to keep it engaged just long enough to test drive it. If your symptoms vanish I'd be pretty confident calling the fan clutch defective.

If the fan clutch is OK, consider the temperature of the upper and lower radiator hoses. Hot water enters through the top of the radiator and cooler water exists from the bottom. If the water temperature of the upper and lower radiator is close, then I'd suspect the water pump or radiator. In your case, you only see a temperature spike in stop and go driving. I'd drive it with the intent of making it run hot, then quickly check it. An IR thermometer would be ideal, but you might be able to get an idea by feeling it (carefully) with your hand.

If everything checks out, then I'd think your cooling system is OK. My next step would be to look for a pressure leak. If you don't own, or don't want to buy a cooling system pressure tester, a radiator shop should be to help you. Basically, it's a radiator cap with an integrated hand pump and pressure gauge. You run the engine with the radiator cap off until the engine is warm. Then you attach the tester and pump it until the cooling system reaches its normal operator pressure. Go have a beer or a latte. If the pressure dropped, look at your all your hoses and clamps for leaks.

I'm sure others will chime in with more suggestions. Just be methodical and you'll figure it out.