This is story related and cannot be missed.
After talking to Kane at the base, drive back to the hospital and talk to Cara and get in the ZDC truck.
Now for the tricky bit - with a damage bar you have to drive to the comms tower in Sunset Hills to pick up survivors. Take it slow and steady, avoid crashing, zombies where possible and you should be fine. The damage bar is restored, and you the have to get to South Almeida to pick up Nelson (the only route being back to the hospital, round past the yacht, onto the motorway and then via the fireworks factory). Take your time, and watch out for rocket launcher troops en route.
You have boost and freeze torpedoes via LB and RB but tbh I found both useless. Also watch out for the red barrels, esp just past the hospital - note you can't get out of the truck. I got out to shoot them and got "game over" straight away! Just push the road sweeper into them and this should blow them all up.
After picking up Nelson, you have to head back to the Diner from the start of DR3, and again just take your time. I made it with about 1/3rd health.
Last point - the acheivement glitched on me and didn't unlock, which really pissed me off. However I went to bed and found that about 8 hours later in magically did, so don't initially panic if it doesn't unlock.
Good luck!