Don't tighten that upper shock nut with an impact gun, even if you could. You can easily destroy the nut or shock doing this. Impacts are great for taking an old shock off you're going to throw away but, not for installs. Use hand tools. It doesn't need that much.
You've gone through lots here but, it sure sounds like a bad shock install or worn shock. any wetness on the shock itself?
You sure it ain't the exhaust pipe hitting the frame or PHB brace? It's possible here when you install axlebacks.
2014 Premium GT, SGM, Brembo, Auto, Tech, Comfort, etc. GT500s, Steeda UCA, VMP auto tune, 100 shot on BBR tune, best et: 11.38 at 120.85
2012 GT, Candy Red, Brembo, 3.73, Tech, Comfort, etc. Procal tune, Roush upper, UMI LCAs, Steeda red bracket, GT500s, 28X10X16 Hoosier slicks, best et: 11.91 at 115.23 RIP