Driveshafts - a bit more...
Long term Lancia Aurelia and Flaminia owner here - the issue of driveshafts to the rear transaxle really starts with the Aurelia in 1950. They refine their driveshafts through the 1950s, and end up in the late 1950s with guibos and two driveshaft tubes, and its this same design in the Flaminia through the 1960s and then in the 1970s-80s with the GTV6 and Milano. Its practically the same shaft. So this issue has history.
The best guy on driveshafts I ever found was Roger Vrilikas, at Drivelines of Oregon. He worked on a few Lancia shafts for me with good success with both theoretical and practical knowledge of driveshafts Ė and he was quite familiar with Alfa GTV6ís. His shop also did big marine shafts. He wrote up an impressive article in the Alfa newsletter about 10-15 years ago about how to get rid of vibration. He believed the problem was more in alignment than balance: that is, things in the driveline get out of alignment in small ways, and send the shaft into vibration. He looks very carefully to make sure each component has its center of mass on the centerline of the shaft axis - typically the problem. There are small variations, due to wear, or to manufacturing intolerances. He fixes that, and finds the balance problems to be much less as a result.
First, they inspected the shafts, and carefully examined each part, making sure it was straight and true. In my shafts, the fitting splines were kicked slightly out off true, as the connection of the spline into the tubes (welded) had given way a bit and been tweaked to be a bit out of true over the years. This is not uncommon, as that connection takes some abuse from fast starts. To fix this, they heated the ends of the tubes, and gently pushed the splined bits into alignment, back on axis. So too, by fiddling and switching all the connecting pieces, they were able to get the entire driveshaft to spin much more straight. They also made end couplings for mounting the shaft to spin the whole thing it as a single entity on the balancing machine, and got the runout from .020Ē down to .006Ē, measured along the whole shaft, not just at one place. Once straight, the balancing of the shaft was very simple. In short, its alignment first, then balancing.
I think Roger has retired, I don't know if Drivelines still exists, and if they still do shafts. But he did a few of them for me, and they were wonderful.
The hose clamp and vibration analysis is a good home measuring unit for vibration and reduction by counter-balancing. Some Lancia folks jury rigged a similar acoustical setup in the 1970s, with a speaker and some sensors, for the same effect. It takes some time, and if the out-of-alignment isn't too bad, it can work.