This mission is given to you by Devin Weston ("D" on the map) and can be started by either Franklin or Trevor. As Franklin, meet up with Lamar and drive the stolen car to the next waypoint, where you need to park it on the truck. Once you do, you'll switch to Trevor and will be in for a long ride to Paleto Bay.
When you get to about 3 miles left on the trip, you will end up getting a 2-star wanted level. You'll need to keep the truck steady while Franklin makes his way to the JB700. Eventually, Franklin will get into the car and you'll gain control of him. The JB700 still has the spike strips from before, but it also got miniguns mounted on it.
Use both of these to take out all of the cop cars that will be in pursuit. Once they stop pursuing you (prompted by Lamar saying they are all out of cars), follow the truck to the designated to finish the mission.