All 2nd gen Rams have plastic tanks. Until around 1998, the diesels had copper cores. About 99 they went to aluminum cores. Probably save money. Also aluminum doesn't cool quite as well. The latter helps DC with all the calls complaining about the temperature cycling. That is caused partially by the huge cooling capacity of the copper radiator vs. the minimal heat put out by the engine under low power. When ambient temp is low, the engine block may provide enough cooling and the thermostat doesn't open. With a bit more power, just enough heat is produced to need a sip of cold water from the radiator once in a while. Thus the thermostat opens and closes, causing the temperature cycling. A "leaky" thermostat is one fix. Another is less radiator. That is another reason why late model owners seem to have more trouble with high coolant temperatures towing heavy trailers.