Having recently completed this game, I thought I would share a tip that might be helpful.
I played through the game without worrying about achievements. I had peeked at a guide and saw that there was a free roam mode after completing the game.
Sure enough, most of the achievements came naturally. After finishing the story, I was down to three achievements left to collect.
Love Turts
The Life and Times of Raccoon Carter
Ol' Shoshone
All three of these can be collected easily in free roam, and they are all close together. Make sure you have the axe before leaving the lookout post, and walk to the North East part of the map. (This is the order I got them, but you could probably be more efficient getting them in a different order.)
The turtle is on a rock near some pine trees near cache 302, north-east of it
South of this, the raccoon is in a stove in the basement of a run down cabin found near a run down outhouse where the trail splits to the east between the Thoroughfare Trail and Telephone text on the map
The cassette is found in a clearing west of the raccoon behind some bushes and some limbs that need to be cleared with the axe
Cheers!