Hi. Just my 2 cents. Check the transmission extention housing bushing that the driveshaft slip yoke slides into for excessive play. I had a 1966 Dodge Coronet that suffered from a vibration similar to yours. Replacing the bushing took care of said vibration.
Willie
LeBaron1973 thank you. Getting back into amateur radio in the past three years has taught me quite a bit about your beautiful country. Best wishes to you as well.