72 350SL Vibration

[MBOwner]

This is my second problem with this car, first one is intermittent stalling, that has me searching for expert help. I get these two fixed and I am one happy man. Again, 1972 350SL with 130,000 miles. I have had a vibration on acceleration and at certain speeds, not necessarily RPM. Mostly between 40 and 65 MPH and more obvious when more torque is applied. I have targeted the driveline as the problem, but could there be something else? I have replaced rear transmission mount, driveline flex couplers and entire driveline (new and balanced). Vibration still there. Question: should there be some end play between driveline ends and transmission/differential connectors? Do you entend driveline until tight and seated in recesses at connection points? Is there a proper sequence (ends first, center bearing next, etc)?