Is this a balance problem?

I recently put some BFG 265/75 KO's on my truck and think I have a balance problem. At about 65 mph it starts - I don't really feel it in the steering wheel, but if no one is in the passenger seat I can see the whole seat vibrating back and forth slightly. If there is a water bottle in the cup holder the water rapidly sloshes back and forth. I took the truck back and got them rebalanced at Discount Tire but it did not fix it. Are the tires out of balance? Is it just the difference in "ride" with the more aggressive tread?

 

You might wanna go to a different shop and hav them take a look at the tires if they balanced it right when they first installed it.Tire machines hav different modes depending on what type of wheels you hav to balance.I always tell them to use dynamic mode on mine.Works for me not all but most of the time.

 

Make sure they use the right adapter to balance the wheels.