I have a 01 dodge Ram 1500 long box. And it recently started a very bad vibration, sounds like really fast thudding. It shakes the whole cab pretty violently. It only happens pressing on the gas from a stop. The fastest I can for is about 5 mph. If I press on the gas very slowly it won’t do it. The drive shaft front and rear seem to be solid. No wiggling, it’s tight. Anyone have an idea on what this could be?