REWARDS - Respect, $5,000
Get into a car and go to the blue blip on your radar to the ambushed van. When you approach the van, you'll see four men on motorcycles driving away with the goods. CJ hops on a bike nearby and gives chase. You need to nab all four packages from the motorcycle goons.
You can get the packages in two ways, by either killing the goons or grabbing the packages when you get close to the bike. The bikes will split up soon after you start chasing them, which means a lot of backtracking for you to get all 4 packages. The bikers who have had their packages already stolen will give chase and try to kill you, but they aren't too much trouble.
About 5 minutes after you start chasing them, any remaining bikers will start going towards a rendezvous point. When and if this happens, haul ass to make sure you snag all of the packages before they reach that point, or you fail the mission. Once you have all 4 packages, head back to the club to finish the mission.