Two possibility comes to mind, The sway bar brackets may be cracked (click on link) or the hatch is not secure. Open the hatch and look at each side. There you will find a plastic adjustment pad that when loosened can be slide down farther to take up any slack. You should also jack the car up using the differential and then pull push on the tires to check for excessive play in/out, fore or aft.
Give those a try and see what you find.

https://www.crossfireforum.org/forum...-cracking.html