Since you've already checked to see that everything is tight, I think it's time to swap tires front to rear and then drive it to see if the problem remains.

A very high percentage of such problems trace back to tire/wheel problems. Remember the SOURCE of vibration has to be something that rotates, loose linkage, bad shocks and things such as that only AMPLIFY such vibration, find the source first.

Even though the wheels were balanced, there still could be wheel/tire related problems such as a broken tire belt, a stiff spot in the tire or a bent wheel. Swapping them front to rear is the quickest and most positive way to check.

Hope this helps,