In the southern part of the region you'll find a checkpoint north of a bridge. A briefcase is located beneath the bridge on the southern side; you should be able to spot it if you move to the eastern side of the bridge (see screenshot).
A road connects a checkpoint near the center with a checkpoint in the southern part of the region. Along the road you'll come upon a couple of shacks - a diamond briefcase is located on one of the roofs. You should be able to spot it straight away. Getting to it, however, is not as simple; next to the shack you'll find a trashed car which may be able to get you high enough to jump onto the roof. A more solid approach is jumping onto the burned out truck at the back and using that to jump onto the smaller part of the roof, and then onto the main roof.
If you follow the road going south-west towards Shwasana you'll quickly come to a small wooden plateau resembling a looking point or somesuch. Moving there you'll be able to spot a trashed car nearby. Next to the car is a briefcase.