Hey I'm back again. I got new rear tires, and had all 4 balanced.. but the shake is still there, still only at certain higher speeds though. For some reason I'm convinced that it's coming from the passenger's side front.. when my wife is in the car I have her put her hand on the floor when it's happening and she says it really feels like it's from there. It's hard to tell while driving as I'm sure everyone knows, but for some reason I'm convinced it's over there.
The other thing that's happening (this is minor and silly, but it might be related) is when I back into my parking spot at work every morning, when the wheel is turned slightly and I'm backing up, I can hear a squealing from that wheel. I had one of the workers stand next to the car while I was backing up and they said it's definitely coming from that wheel.
I don't know if that helps with what the problem may be or not..