Hello Everyone!
This can be accomplished in ACT 1 Chapter "The Crossing." Leave the safehouse and continue in the level as planned. After shooting the giant propane tank, creating a hole towards the George Washington Bridge, stay to the right side. Continue along the bridge until you eventually see a sign that reads "Kanowa River." Underneath you will see a barge trailer and the Golden Skull collectible is in the far left corner on the ground. Please see the attached video if you prefer visual representation. Good Luck!