While changing the rear driveshaft ujoint, I managed to take a chunk out of the yolk while pressing out the cup. Apparently both "ears" had been dented slightly. So I used a grinding wheel to remove the bent metal so the other cup would press out.

Anyway, I sadly had to use this yolk. My question is, is the balancing on these precise enough to explain why I still have some shaking going on, once I hit 55+mph.

 

If the vibration is from 55mph and up, my guess would be a tire and/or wheel out of balance.. The drive shaft should shake at a much slower speed like maybe 20-30 mph.

 

You can use a hose clamp around the driveshaft to change the balance. If the vibration gets worse or gets better, you know it is the driveshaft. I would expect any vibration from the driveshaft to get worse with increased speed.