Well, you are now at the point where you need to stop driving the car until this issue is fixed. As mentioned before, lift up the carpet on the drivers and passenger side, as well as the rubber / foam noise insulator pad under the carpet. If there are holes already drilled into the floor, it means the recall had already been done at some point the dealer wont fix this. If it has not been done, search around the forum for the actual recall number and call the dealership again.
Either way, the parts are not that much money:
XF1Z-5A364-AA (Subframe bushing recall kit) - 1 kit will cost you $12 to $15 at a dealership (you might want to buy 2 and take care of the fronts as well). They come with 2 bolts, 2 large washers and 2 nuts that go over the tabs of the body/subframe that allow you to tighten the nut.
Aluminum subframe bushings are $75 to $90 for a set of 4, I really recommend them. You can also put Gen3 subframe bushings on the car, not 100% sure of the part number though.)
If you are super poor, you can buy some hockey pucks and drill holes in them for the time being instead of buying new subframe bushings. Just make sure to put the washer from the bolt recall kit below the bottom hockey puck.