My 2003 impala 3.4 cyl. 4speed automatic, shifts the final D, hits a bit hard

My gut is that you are experiencing normal internal wear of a very aged transmission. Internal hydraulic components and clutches are problem worn / contaminated to some degree, causing harsh shifting. I wouldn't even recommend tear down at this mileage, and I don't suspect there is any "easy fix". It may be time to consider replacement of the unit; certainly consult a local shop for an opinion first.