What causes your 1996 dodge caravan not to heat up at 55 mph but heats at 35 mph?

sounds like your thermostat is missing or frozen open so the coolant cant heat up. It needs to stay inside the engine longer to stay warm and with it freeflowing through the radiator the faster you go the cooler it gets in cold weather. The opposite will be true in hot weather, it wont cool off well enough because it wont stay in the radiator long enough.