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This may seem daunting, but there are only 11 combo vehicles and many of them are unlocked through natural story progression. Only one is unlocked from the skill tree. The rest are found in the environment just like regular weapon blueprints.
Mobile Locker: Van + Van
Junk Car: Sedan + Muscle Car, Sedan + Sedan, Sedan + Forklift
Turret Rig: Steamroller + Sedan
Mini Bike: Motorcycle + Sports Bike, Motorcycle + Forklift
Forkwork: Forklift + Fireworks Van
Rollerhawg: Motorcycle + Steamroller
Junk Truck: Van + SUV
Party Slapper: Street Cleaner + Party Van
Junk Bike: Motorcycle + Sedan, Motorcycle + Van
Shockdozer: Backhoe Loader + Ambulance, Steamroller + Van