New "18" Ethics on a 05 Taurus.

My rear tires are wobling when I hit 40 or 50 miles an hour, is it beacuse the adapters are put on woring? or The rims are bent, or I did not have them balanced? When I'm driving I can see the rear wheels wobling and it makes my stering wheel shake also.

P.S. could it be the tires too? H.E.L.P.

its probably the balance