Nissan Maxima shaking at highway speeds

I have a 2003 Nissan maxima and at first it used to shake at around 50mph, took it to the shop they looked into the suspension, did wheel balance, changed 2 front tires and wheel alignment and they said it should be fixed. A week later, it started shacking at around 70 mph, what else should I do to get ride of this problem?

how many miles do you have?