1980 300cd vibration at 40

[Warren]

I have a 1980 3000CD W123 non turbo. I changed the engine with a lower mileage 617 non turbo engine. After that I have a vibration that starts at about 40 mph and smooths out at around 55 mph. I have had the drive shaft completely rebuilt. New "U" joint, bushing in the ends of the shaft. New center bearing and new flex disks. The tires only have 2000 mile on them and I have switched the fronts to the back. There has been no change to the vibration. Just to make sure I swapped out the differential. During the engine swap out, I changed the torque converter, from the old engine which had no vibration. Is the torque converter balanced with the flywheel? Does any one have any ideas? The engine feels smooth when cars is in park and raising the RPM.