1st off any thing put in the inside the shaft is going to cause more problems then it solves . Centrifugal force will bit your butt at relativly low speeds .
I have built shafts for the ZJ and shakers Blazer . nevether has any vibrations at road speed .
Make sure your u joints are timed when you put it together . Ujoints must run together not a 1/4 turn out of sink . That is the number 2 thing i seen cause vibrations on a shaft .
U joints have a hormonic , they need to cancel each other out . note how the yolks are placed in relation to each other before seperating the shaft .