North-east of Ranger Station, on the northern side of the river, you can find a small opening in the cliff. Inside the opening to the right you'll find a briefcase sitting on the ground.
On the western side of Post Office you can find a collection of burned out cars; there's also a section of a large pipe here. Inside the pipe sits a briefcase.
North of the western checkpoint, on the opposite side of the river, you can spot a small hut in the hills. At the hut, beneath a table, lies a briefcase.
South-west of the western checkpoint, out in the open plains, you can find a small shelter with a well besides it. The shelter has a briefcase and a Jackal Tape underneath it.
At Ranger Station, on the south side of the river, you can find a small shelter near the cliffs with some crates beneath it. On top of the crates you can find a diamond briefcase. If you have a hard time actually getting the "grab icon" to show, try to jump and press the Use key.