If your tires "smooth out" around 55 its most likely because you got some heat into them and the flat spot is now gone. My boggers were horrible for this after i would park aired down. The colder the temp the worse it was and the longer it took for them to get round again. At first you would think everything was going to rattle off the truck, after 5 miles down the road at 35+ MPH everything was smooth as a radial tire.
Basically I think you are wasting your time with the bb's. It sounds like the tires are balanced pretty well for a bias ply since you say they get better with higher speeds (really just more time on the road building heat). If they were out of balance it would never improve and the vibrations would get worse with speed.
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