Springs and diff tightness arn't going to cause your problems. If your drivetrain is free and there is no binding, your car is tweaked. First check your shocks. Make sure they are built the same left to right. Tear them all the way down and rebuild them, replacing the o-rings and pistons. Build them to the same lengths by measuring the extended lengths. Check the rebound too so they are as close as possible. Make sure that the lower spring retainers are the same left to right and start with the collars in the same positons. An easy way to do this is to count the threads from the end of the shock body to the collar.
Now install the shocks and disconnect the sway bars. Now put the car on the setup station. Set the ride height first, then the toe and camber and droop. Then check for tweak. Now reattach the rear sway bar and check tweak. If the tweak is off adjust the sway bar till the tires lift evenly. Repeat with the front. This is the only way to get it set up correctly.