I am sticking with something outside the transmission because the tolerances on those shafts are so small that I don't think it could be moving that much and still function....
But the driveshaft is effectively suspended in RUBBER... and weighs much more than your 3 rd and 4th gears.... and thus has a much better chance of being able to produce this vibration...
So put the Redline in , feel better because it is great stuff to have in there, but don't be looking for that to cure this vibration problem....