Does it go away if you lift off the throttle and push in the clutch?
Can you make the vibration vary with the throttle?
Is it more profound with harder acceleration or deceleration?
Does it seem to follow engine speed? Does it do it in second gear the same as hi gear?
Or does it follow road speed? So maybe a tire?
Need to pinpoint the source, have someone climb around the car listening for where it may be coming from. (front? rear? right? left? etc.
A tire or wheel (even a wheel bearing) vibration has a different tone than a drive line vibration.
I had a drive line problem a few years ago, (actually chased it for a few years) I had it balanced by more than one place and was deemed good, but still had the vibration. I tried changing the pinion and engine angles, even tried a different transmission and a different third member.
I was sure it had to be the driveline. In frustration I had a new driveline built with all new everything and the vibration went away even though it had had the balance checked by experts several times.
My theory is that the experts only balance the drive line when it is perfectly straight, not putting the U-joints at any angle like in real use. Never did determine the exact cause. But you can be sure that this particular driveline doesn't exist anymore.