I have always done it with the bumper off. The rear bumper comes off in about 10 minutes. After you remove the covers over the taillights in the trunk jamb and unplug the wiring, there are only about 6 screws holding it on in the wheelwells, the rest just push clips tight on the 1/4s and the rear body, no big deal. I guess you could get the diffuser off with the bumper still on. You can see most of the clips around the diffuser if you just poke your head underneath. The clips that hold it on are very weak and prone to breaking. Be careful. The ones along the top edge will be the hardest to get out without damage.
