Hi.
The reason it vibrates at a certain speed is when the wheel/tire imbalance reaches a resonant frequency and vibrates. In or out of this speed not so much.
If you think your tires are balanced because the guy at the shop said so, be skeptical. Ask him when he calibrated the balancer last. Ask if he checked that your rims are straight and round before balancing. Did he remove all the weights, and any gobs of tar, dirt, oil that may have accumulated on the rim. You can balance a bent or out of round rim, but piling on more weight will not fix it. Don't get me wrong, some shops are really good, but the trick is finding one.
I have a lot of cars and always seemed to have trouble with vibration, not to mention the hassle of bringing cars or wheels to a shop. So I bought my own Accuturn dynamic balancer, and a manual tire changer, since then no more vibration issues. No more hassle dealing with tire shops either.
I have attached some information from the Accuturn user manual that explains more than you probably want to know about balancing.
Thanks,
John