Rear subframe, rear subframe, rear subframe! LOL
That's the place to look at the most, especially where the diff attaches to the trunk floor (ala single ear). Due to a loose rear subframe bushing the rear subframe causes the trunk floor to tear. The best fix for that is to install urithane subframe bushings which reduce that stress. Especially on the more powerfull 3.0L.
Good luck and hope it works out well!