I would make sure its not the wheels or tires. Try rotating and see if it changes the problem. Or, visually check this is by running the car on a lift at the speeds you have the problem and see. (consider safety first!) I had a rear left tire hop all over the place while the right side appeared to stand still it was so smooth. In my case it was actually something in the wheel carrier itself (bearing or flange - I don't know, I replaced the entire assembly) but I think that's a rare case. When the car is going uphill weight transfer will make rear problems more noticable.