dodge 3500 cummins clutch fan engages on flat with no load

Go ahead and replace the thermostat, but my bet is on a partially clogged radiator. Either by external trash in the fins or the water passages clogged inside restricting flow. The fact that it climbs with speed means simply that the engine is making more heat than the radiator can get rid of. Normal ram air effect at speed should cool it with very little fan needed.