The Bigger the Better
Very rare1.5%All-the-thingsOptionalAnywhere
Special Transport
Complete delivery of all oversize cargoes (requires Special Transport DLC)
There are 10 different oversized cargo types in the DLC - just complete one job of each kind. You'll get 
The "Haul Truck Chassis" cargo will also unlock 
Oversized cargo appears in the job list with red/white striped line down the left side - its hard to miss them, but you might need to travel to get to a departure city to take a job.