A lot of bumper manufacturers use the bolt holes that are used to bolt the factory tow hoops for the bottom of the bumper mount and then use the two bolts that are in the rear tail gate jam for the upper bolts. It seems to work OK but I'm not a fan of tying the chassis directly to the body without any rubber/poly. Vibration and a ripped body and/or frame mounts seems to happen over time especially if you are doing a lot of flexing as part of your wheeling regiment.
Good luck, Eric