93z w/ stock suspen. I recentaly put on a set of 18" tires. I have noticed that it sits a little higher than before. Now I have also noticed that when I get up to 120-130mph that the *** end seems to want to sway from side to side. Is this due to more getting under the car? If so,can I lower the car or change to a different susp. set up or both?

 

Did you match the outside diameter of the tires you used on the 18" wheels to the stock tire diameter? If you did, there is no reason the ride height would have changed. The shorter sidewall will definitely make the suspension "feel" different, but it shouldn't affect ride height if you select the tires properly.

 

When I changed out the tires/rims I did not do it properly.I changed out my rear end gears so my speedo was off anyways so I didn't care if the o.d. was the same. All I wanted was to fill in the wheel wells. I could have went w/ a thinner side wall but the ride would suffer. So the q. is do I lower the car or change out the suspension, or install a kit that does both?

 

Put the right size tire on the car, one with the same outside diameter as stock tires. If you want 18 inch tires you're going to pay the penalty in ride quality, or have the issues you have now.