Proper alignment is the best route.
I'm not suggesting that the value of these products is dubious or suspect, simply that a vibration indicates there is something out of step. The products you mention are not designed to eliminate vibration per se and as such may also come under undue stress by whatever mis-alignment is causing the vibration in the first instance.
Generally speaking, engines/gearboxes are mounted on easily adjusted up/down and left/right mounts and unless you have a failed mounting (which should be replaced first) the alignment from original can't be far away. Given that these drive lines are rigidly connected, it takes very little mis-alignment to cause a vibration. Here's what I would do:
- Undo the propsaft coupling off the gearbox and move it back to inspect the inner surfaces. The bolt holes may be burred which could be the problem.
- Move the shaft back into position to leave a small gap
- Choose four diametrically opposed points, alongside the bolt-holes is a good start.
- Insert a feeler gauge (or any other thin blade) at any one of the points and pull the coupling together against the blade
- Now check the clearance at the other points by inserting the blade at each point in turn, trying not to move the coupling. This will show up any misalignment
- Adjust the engine mounts until the feeler guage goes in with the same level of resistance on all four measuring points.
- Reconnect the coupling
If the vibration still exists, start looking elsewhere for the vibration. Could be a damaged prop or a bent shaft in which cases nothing that you add into the drive line will help. Running the shaft in gear and holding a pointer against the shaft will soon identify a bent shaft.
The prop will need a visual inspection but be aware that damage to a prop is not always glaringly evident. It could be a bent rather than "injured" blade.
This process is easier than it sounds, give it a go, it'll save you problems down the line if you get it right and IMHO is better than add-in solutions.