Bigyellow, I think the answer lies in that the balancing " took care of some of it".
I got quite a dissertation from Denny at Denny's Driveshafts, my stock shaft was beyond balancing so he built me one.
Most shops don;t have the equipment to allow them to balance at above 2000 rpm. It's quite possible that your vibration, possibly up in the yoke area, isn't readily apparent below that rpm. So it was a partial solution.
Either find a better shop in your area or call Denny's in Buffalo. ( Or a replacement shaft from a yard, which I couldn;t find for my 67).