Traditionally vibrations are caused by rotating assemblies that are out of balance. I’ll say first that not all tires are created equal, and that could be the source of the problem. You might start by having the tire balance checked again. Even though they were just balanced, one of the weights could have come off causing the issue.
If the tires are good, then it might be time to look at the driveline. Loose U joints or a driveshaft that is out of balance can cause an issue like that.
More info on solving vibration problems here: http://www.ericthecarguy.com/faq/determining-the-causes-of-vehicle-vibrations
Good hunting and please keep us posted.