Before you waste your time and money on an alignment...

First, I would have the balancing done again... It's recommended that you rotate and balance every 5000 miles... Check the air pressure and have them take a look at your wheels during the spin balance to see if any are bent.

Then, have somebody shake down your front end... Check ball joints, tie rods, and so on.

Finally... The first question i ask is whether or not it does this when under load... If the car shakes with your foot off the gas, it shouldn't be tranny or anything like that. If the car only does it when you are under load, then you might be looking at a drive train issue such as the axles.