Ayup disaster on wheels the minute they hooked it up.
Most people think that an Excursion is the same as a F250, nope the rear end is '"softer", different spring rates for a more comfortable ride. Known issue if you do your research.
I this case speed and the tail wagging the dog appear to be the main issues, see it every day in my travels. Watched one for several miles the other day, he would get up to a certain speed the oscillations would start, he would slow down, then start the cycle again, it had Michigan tags on it and I saw it in VA. I wouldn't have driven it that far, without fixing something.