The 'Quadra-Shock rear suspension' include all four dampners, not just the horizontal axle dampners.
You have helped with my shock absorber education. After doing some googling, I just learned that a "damper" is the same as a "shock absorber".
And, yes, I think I understand that I would need two quad shocks and two regular shocks to do the rear of my Mustang.
I just ordered the Gabriel 14039 rear quad shocks, as suggested earlier by 'Mustang5l5'. Amazon had by far the best price.
Now I just have to figure out which regular rear shock absorbers to order.