I equate 'rig' with Trucks and SUV, just used hand in hand, synonymously if you will.
According to Wikpeida, it's a slang for a Truck and SUV.
(10th bullet down.)
Also Rig was a Norse god! 'Described as old and wise, mighty and strong.' I'd say that would describe my D-90.
Btw...the computer geeks also call their tricked out pcs rigs as well.