If your set of rims needed rings due to the difference in bore size, the rings should have been put on all four of the rims. It is possible that the rears are the ones shaking at the mentioned speed due to the lack of the hub-centric rings.
Try checking your rear tires too (Bad wear/Balance)... the rears are usually worn on the inner portion of the tire most especially if you are using after-market springs such as Eibach, Vogtland or the like. I have a set Eibach's with a staggered set of Style 32's and they ate up my last set of tires pretty bad. But then again the ContiSportContact's are not known for good tread wear.