Before I lose you with my long story, does the vibration come and go if you maintain a long curve on a highway? Will it sometimes be stronger, and then fade out, then later come back?
Being a tire monkey myself, I can say a few things about balancing wheels. While it isn't rocket science to roughly balance a wheel (well enough for a truck or big van, something with heavy ass wheels), getting it perfect for a more precise application can take a bit of skill, and the right balancer.
Here follows an overly descriptive narration about wheel balancing:
Your typical balancer will display the amount of weight that has to be added to the inner and outter lip of the wheel to bring it into balance, in increments of 0.25 oz. Getting the machine to say 0.00/0.00 is easy, but that doesn't necessarily mean the wheel is perfectly balanced. Some of the fancier machines will show a third reading, called "Total Static". This reading indicates the total single point weight that the wheel is out, and it reads in 0.01 oz increments. It is possible to have a wheel "zeroed" and still have a total static measurement of 0.30 or 0.40 oz, because the machine doesn't have anywhere to put a full 0.25 oz weight.
I had this problem when I mounted up the tires on my BBS rims. It was a busy day in the shop, so I got one of the other guys to balance them while I mounted. He was competant, but nothing special. Sure enough, first drive the car shakes from 60-65 mph. I come back, pull the wheels, and start re-balancing. The guy is ranting at me, and sure enough, the inner and outter lip measurements are zeroed. But! The total static was at least 0.3 for each wheel. To fix it I had to cut the weights down to smaller sizes than they come as and stick them on individually. Got 'er down to about 0.05/wheel.
Smooth as glass.