Spoilers for those that need to know this. I finished LR without finding all the warheads or Ralphie posters. I’m going to go back to my save just before I walked into Ulysses Temple and take care of “unfinished business” and try and track down the warheads and posters. The warheads shouldn’t be too hard as I’ve found a guide to all 30 locations. After the Temple I had found 27 and I think I can get those last 3 at the Courier Mile which I hadn't done yet. The Ralphie posters are going to be an OCD nightmare as just before Ulysses Temple I had found 13/20. As I retrace my steps for the warheads I figure I’ll keep an eye out for the posters and this will be hard because you can’t tell if a poster was “read” or not until you get close enough to “E” check it out. Meanwhile I’ll keep my eye out for a poster location guide if someone ever makes one but so far i haven't found anyone who has found all 20 Ralphie posters yet. This should keep me busy until Gun Runner's Arsenal comes out.

Link for the warheads http://www.xbox360achievements.org/forum/showthread.php?t=330752.

Why would you go back to an old save you can go back and look for everything and whatnot.

Yep, that's the guide I found. I bet the warheads I missed are in the Courier Mile (2) plus the one you have to clear to get to the Courier Mile. I may have detonated the entrance one though which would leave one odd warhead somewhere, probably before the cave of Abaddon or that big canyon before Uylsses Temple. The Ralphie posters are going to be a challenge.

I know this works for the warheads and the achievement but what about Ralphie posters?

Not sure, but I don't see why not.

The only reason that I say this is so you don't have to go through that pretty long end piece.