I just replaced the transmission (116,000). Then came four new tires (117,000). Tires weren't balanced properly, so back it went. Initial drive-out and I thought it was fine, but once back out on the highway at 55-65mph, there it was. Vibration in the driver seat that feels like a massage chair has been installed. Get to around 68mph and it's completely gone. What in the world could this be?

check to see if the axles(half shafts) are installed properly

Thanks for the input, I'll have that checked out! Right now I'm thinking I may actually have 1 or 2 bad tires. I've had them balanced just in case that was the problem since that would be a really easy fix - one of those too good to be true things probably.