Well, although it is widely called a "strut", what the rear of a 2007 Ford Five Hundred uses is a "coil-over shock", such as the Monroe® Sensa-Trac® 71120
. A bag-in-coil is out of the question, and an air-assisted shock is tough because it must carry the specific lower mount for the coil. Monroe says it is used only on the 2005-2007 FiveHundred/Montego (the 2008 Taurus unit is a bit different in dimensions), so there's little hope for finding something for a similar model.
I agree with the earlier suggestion that if more spring stiffness is needed, the practical solution would be replacement coils; it is unfortunate to give up the adjustability of air.