Whats wrong with my van

I have an 07 kia sedona.I had 4 new tires put on an a wheel alignment. After alignment I still had vibration when driving over 40 mph. Vibration would get worse when accelerate. They thought I had bad tires so they replaced tires with 4 new ones. Checked alignment and balance and I still have vibration at those speeds. Local mechanic says suspension is all tight and everything looks good, what else could be the problem.