If you want to keep the tires, find a shop with a road force machine.
Hunter GSP9700 Wheel vibration Control System solves wheel vibration and tire pull problems that balancers and aligners can't fix
Will probably cost you about 90-100 bucks, but this machine will actually measure the roundness of the tire, runout of the rims, etc. Basically if this machine can't balance a tire then the tires are defective in some way. If there is a high spot on the tire, this machine will show them how to rotate the tire on the wheel so the high spot of the tire matches the low spot of the wheel for a more uniform ride. Make sure you find out what the highest road force measure is after then balance. Try to keep them under 10 lbs. I am very picky about any vibration at speed, and I can tell you that it seems now more than ever I have been having problems just using a regular balance machine. these large wheels are helping (20") but I think tire manufacturing QC has gone to crap lately.
Tire balancers only tackle the balance of the wheel/tire combo, and when you are having issues after a balance you have to go this route. Key also is making sure the shop you use actually knows how to use it.