For the rear, it is almost easier to build a new rear bumper that is strong enough to do what you want. If you just use a piece of 3/8 plate with a hole in it that you can attach to the frame, and extend it through the rear bumper it would give a strong enough spot to attach a shackle to. You would want to find a way to brace the two ends of the frame together as to spread the load, and help keep the truck from racking, or twisting.
I would use shackles and tabs instead of the perm, mounted rings. Rings like that are noisey and you need to use a shackle to attach to them anyway.
You can also just bolt a tow hook with strap retainer to the bottom of the back rails to accomplish the same goal.
For the front you can use the factory locations to mount hooks to.
I have seen people mount the weld on rings to the cross member that is behind the bumper cover, but that is not something that I would recommend. That cross member is the actual bumper, and it is only held on with two bolts at the end of the frame horns. And the ends of the horns are at the end of a crumple zone. If you give that cross member a good yank it will bend and take the ends of the frame horns with it.