If you've replaced the shocks and are still getting a slight vibration you may not have corrected in time before it caused a odd wear pattern on the tires. If that is the case put the front on tires on the rear and see if that takes the slight vibration away. If it does you'll know that the tires have a uneven wear. Really not much you can do for them at that point. I have heard that some people have put them on the rear and did a good burn out with limited results.