OK so you did lowering springs and upper and lower control arms. How many miles are on the car. It could be a few things. But I'm wondering if the bushing that are in the rear for the upper control arms are shot. Could be a bad shock or quad shock.
It could also just be the road. Now that you have new stiffer parts you are feeling the groves in the road. Try to find a street that has just been repaved and see if it still does it.