No fox body has leaf springs. The term Fox body is synonymous w/ coils on all four corners. Show me an example of a 90's fox body w/ leaf springs, and I'll show you a car that is mistakenly labeled as a foxbody.
If you have one side lower on your car something is either worn out or broken. Mustangs get beat on and torque boxes fail. when the floor starts to tear where the mounts are, the chassis sags. If a control arm bushing is shot, the car doesn't sit right either. Lastly, the coils themselves just wear out. Typically the passenger side coil has had to deal w/ torque transfer it's whole life,....it wears differently than the driver's side. The axle tubes "pull" forward on the rear end..I've seen an 8.8 out of several mustangs where the axle tubes are so visibly bent forward that the rear had to have the tubes heated and pulled back to straight. When a tube pulls,...It preloads the chassis like a ladder bar does on a race car,...preload causes one side or the other to raise.
The reason that there is so much info out there about the car sagging on one side is because they are over 30 years old, and by nature have more than likely been beat on at sometime in the car's past. And the reasons for the above are all attributable to one of the three.
Lift up carpet, check that your torque boxes haven't started to tear out. Look at the floor mid point to be sure that a tear hasn't started there as well. Replace the
control arms, or the bushings, change out the springs w/ new ones and install SF connectors if they are not there already.
For god sakes do not raise the car up w/ a stiff spring or a freakin airshock....It'll ride and handle like ass, all while looking stupid at the same time. The 70's are dead. Gabriel High-jackers died right along w/ N 50-15's, Boston,..and leisure suits with them.