Your tire pressure is fine, my bet your stiff ride is caused by nothing more exotic than the shocks included in the kit. Shocks and tire pressure are the two main factors in how a Jeep rides. Swapping the shocks out to a better riding model is my suggestion. One shock that is moderately priced that has a great ride, from personal experience, is the Rancho's newer gas-charged RS5000x... not to be confused with the older rough riding hydraulic RS5000....and rides stiff like my old lifted YJ.