Originally Posted by Weasel

Well, the main reason the OEM bushing was designed the way it is would be to absorb some of the impact on the steering/suspension during driving. It is kinda like a small shock absorber built onto the thrust arm.

The solid polyurethane bushings transmit all of the feel from the bump through the steering straight to the steering wheel. That is one of the given side affects of the poly bushings.

For the slight shake while driving, it most likely was always there but you couldn't feel it with the OEM bushings. It is most likely caused by wheel balance and may be worth having your front wheels road force balanced. (and make sure they actually road force balance, tell them you want the before and after printouts for each wheel)