Since it always had the vibration from the day you purchased the car it does change things a little. Someone in the past may not have indexed the driveshafts properly when replacing the centre carrier or bearing. Or one of the shafts may even be bent.To prove imbalance you might want to put two screw clamps on the driveshaft and rotate them to different positions to see if vibration is reduced or perhaps get another intact shaft from a salvage car if cheap enough. First again though you want to critically check centre universal and rubber end disks etc. You can check for bent shaft on car though quite easily. Shame to have to pay the bucks out to have the shaft balance checked. As problem did not develop in your possesion it may be required. Know it bugs you as you mentioned you had a dynamic balance of all wheels done already to try to eliminate the problem.