From the Menu--Takeovers--Warehouses--Hijack (20 minutes) --Easy--
If you are a level 50 or near, you are no doubt an expert at this by now. If you've never hijacked a truck before press , shoot the back of the truck a few times and 2 gangsters will pop out and shoot at you. Kill them and talk to the driver. It helps to kill the driver after you get the keys, so another truck can re-spawn shortly in that same location. Drop the truck off at the nearest drop off point identified as a 'blinking brown truck' on the map. Continue to drive around, park in front of a truck once you've found one, and hijack until you've hijacked 5.
An easier/lazy method is to bribe cops to the east of your spawn point down Grand St. in front of the 'The Bowery Bank'. A truck usually arrives at this intersection and the cops will help you kill the mobsters. One you've hijacked it, drive it back to your spawn points' garage, or any of the truck drop offs on your map. Note: Driving into a truck will make it speed off, thus making it harder to get.