Try something like "Air Lift Dominator" air lifts, they are about $70 on Amazon and don't need to be bolted in, just jam them between the control arm and frame and maybe wrap some wire ties or bungees to keep them from popping out on the road. If you tow regularly, go ahead and bolt them in. They work.
if you are pushing the rear of your car down that much you have too much tongue weight. you either need less trailer or more tow vehicle to tow right.
Looking for a Used G6 in your area?
CarGurus has 3,839 nationwide G6 listings starting at $1,250.
Search Pontiac G6 Questions
Pontiac G6 Experts
Related Models For Sale
Used Cars For Sale