2007 Mustang GT has Vibration at 70-75mph

I used to have this vibration issue and always thought it was wheel balancing. Then it seemed to pretty much disappear. I had to get 2 new rear tires and it was back with a vengence. So I had the 2 rims in the rear re-balanced with a Road Force machine and it was much better.

But I still have a bit of a vibration at 70-75mph. I probably should have had the place re-balance all 4 rims. I am going to have the front rims done on Wednesday.

If that doesn't solve the problem I will try rotating the tires and see if that has an effect.

I Googled the Mustang 2005+ Vibration and found a thread where someone said their driveshaft was out of balance. What are the odds that could happen ?

Could there be other reasons for this vibration like alignment, shocks, etc ?

My car is almost 4 years old with 33,000 miles and I don't believe I have ever had an alignment, but then again the tire wear doesn't indicate that I need one but who knows ?

It's not horrible but can be a little annoying.