My Taco has a similar problem -- 265/70 - 16 BFG AT KOs.

This set of tires has been balanced at least a dozen times on five different balance machines (including match mounting on two different GSP 9700s), and always come up zero after the tech adds the required weights. The rims and tires run absolutely true, and are smooth as silk up to 60 mph. From 61-62 up to about 72 mph, they will shake the fillings out of your teeth. At 74 mph, they smooth out again.

I've come to two conclusions:

1) my BFGs suffer from excessive radial force variation (sidewall stiffness is way different from one side of the tire to the other -- as much as 39lbs on one of them), and

2) My Taco's suspension is sensitive to the vibration harmonic in the above speed range, and amplifies the shake instead of dampening it.

Good luck, as I believe the only fix for my problem will be a new set of tires (BFG won't do anything about my current set <img src="/forums/images/graemlins/barf.gif" alt="" />).