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..