Cant help you much with the isue,except to confirm that these cars are not supposed to have a rough ride,my 2000Durango 4x4 with a 360 is a nice and fairly smooth ride,not as smooth as the Benz on highways that it replaced but certainly far far from a rough ride.

Keep at it and replace worn out parts and look for worn our or even missing bushings and balljoints and im sure you will find the problem.