NOTE: I did not post this because I am trying to diagnose the problem. I posted this to help others who may be experiencing a similar issue and . It isn't the only thing that causes shaking of course, but in this case, it was caused by 3 bad tires and bad alignment.
This is on a 2003 Mazda 6 with over 215,000 miles. The vibrations start when it reaches 45 mph. Two different shops told me it was the alignment. After filming this, I had the alignment done and replaced the 3 tires that had worn unevenly from the bad alignment (the 4th tire was still new). That resolved the issue immediately.
