Did this problem gradually come on, or all of a sudden? Did you replace the front coil spring? Jack the car up and do a thorough eyeball inspection underneath looking for any damage, rusted out frame rail, separated welded joints etc. Take a hammer under there and tap the hell out of all parts having anything to do with the integrity of the rigidity of the frame as such. Does this Mustang get driven in snow country during the winter? If so, you may want to look very carefully for massive rust.