If you have an 8.8 rear end, you don't need to change it out regardless of if you get a 4.0 or 5.0. If you have a 7.5, you need to change it out for an 8.8 for either motor. It's much stronger.
For the trans, you'll need to swap if you get a 5.0. Won't have to swap with the S/C 4.0, but like I said, you may want to consider beefing it up some depending on what kind of power it's putting out.