The flex disc/guibo will only happen during initial acceleration.
Why? Becuase of physics - when the driveshaft is spinning fast enough, due to angular momentum (spinning). Think about a top - you spin it and it stays in place due to the outward momentum around the entire thing (Or a CD when it's being played.) Now if you take something like a CD and it is unbalanced, it won't work right - it'll rattle to death.
If you have vibrations at higher speeds, I would check something out the bushings.