That is weird! Sounds like maybe a sticking strut - not good at all.
Or, check the sway bar, sway bar link, sway bar bushings (where the bar mounts to the body). There is a *very* slight chance that something is broken and your getting binding.
For older Stangs that see winters, the bottom of the spring can RUST to the FCA and that will prevent the spring from fully flexing at the FCA. Then, the spring can break right at that point. That's what happened to me, and I got another Fox spring from someone that did the same thing.
But, even then, at worst, it would add height as the broken part sat on top of the rusted in piece. When you said 4x4 then not, that was the one thing that came to mind.
But, strut bind is *much* more common!
When you're doing the
suspension this winter get the new FRPP FCAs. They have the new "forever" ball joints, and slightly stiffer FCA bushings. Also, get at least a poly isolator for the bottom of the spring. The OEM rubber compresses over time and that's what causes the most drop in the fender over time
If you want a firmer ride but not much drop, go with the Mach1 (~1/2" drop) or Bullitt springs (~3/4") drop. But, there is a 1/4"-1/2" difference between cars depending on a number of factors (alum heads, AOD, AC, 3 core rad (more water weight), etc).
I run the Mach1 springs, poly isolators, and a 1/4" Steeda spring spacer (to give me an ~0.4" fender rise_. With my AOD, AC, 3 core rad, POS heavy *ss iron heads, my car sits at 27" (vs 27-1/4 ->27-1/2 spec). The springs are ~50% firmer than stock (600 vs 425), but the car handle awesome one the roads.
I have a bunch of info at:
http://veryuseful.com/InvisionBoard/index.php?showforum=2
But, I need to better organize it.