If it occurs through 40-50 mph while cruising or accelerating it is possible it is tire balancing. However, if it is just when accelerating I would agree that it is not a balance problem. Usually, this is typical. Once when my tires were out of balance, there was no shaking. However, between 65-75 mph it was quite noticable. Before and after that interval it was fine. Is the shaking coming from the steering wheel? If so, it is probably one of the the front tires that is out. If not, it is most likely a rear tire.

Hope that helps