Dragging cans

hello, all. Took the 65 valiant for a 1000 mile road trip last month up and down Californian. She did great. Only thing is since I've been home it now sounds like I'm dragging a tin can behind the car. The noise happens when the car is on the ground rolling- in gear or not. The noise does not happen when the car is on jacks, in gear, wheels turning. Everything feels mechanically sound while driving and rotating the rear wheels by hand. No obvious grinding or looseness. No shavings in the differential fluid that I could see. Seems to be a weight vs rotation situation. Is there an obvious problem I'm not seeing. As always, thanks again for you input.