Are you planning to remove the rear bumper for the repair or are you going to do it while it's installed?
Any pics of the damage?
For minor to moderate cracks, I normally like to remove the bumper(s) for repairing since it gives me full access to the back, where I usually use a small amount of fiberglass and resin to bond the damage. Once dry, I go to the exterior and fill the remaining crack with spot putty and lightly sand to level it out. I'd then apply touchup paint and clearcoat to the exterior surface, wait until dry, then westand with 1,500 and 2,000 grit and buff with a good polish until the damage is as invisible as possible.
Then re-install the bumper...of course.
On my old '03 Envoy, my front bumper had a few cracks around the drivers side fog light and it did not firmly stay in place. It also made the fog light housing vibrate while driving. Annoying!
I did the same technique with the fiberglass and resin repair and it worked perfectly.