I have the same or similar issue and have been dealing with it for quite sometime with no success. I have been throwing parts at it left and right, I'm down to one last thing before I take it to the dealer and let them deal with it for me. I just bought a hellwig sway bar and end links, I will let you know if it helps.
2012 mustang v6 performance package, power mods: airraid intake, borla s-type exhaust, appearance mods: saleen front grille, hood spears, roush side spliters, 2013 tail lights, retro fitted head lights with led and hid, shorty antena, black deck lid, gt500 steering wheel, oem take off leather seats, retro black shift knob, suspension mods: koni shocks/struts, sr springs, gt500 top mounts, panhard bar, lca.
1994 mustang gt 5.0