Overloaded? Watch to the end. :)

I think the entire stunt is rigged. If you look carefully at the truck at the beginning, the body is already bent in the middle. When it get's straightened out again, it looks like it's bent the same way, no more, no less ... like its on a hinge. Driver might be inside releasing a pin. If it was really bent THAT much, the drive shaft would be bent all to ****! not to mention the damage done to the transmission. Nah! This is rigged. It's a great stunt, but still rigged!

And yes, the weight in the back of the pick-up would have advantage over the monster truck. Probably, if the pick-up was just pit in park with the brakes on, the monster truck probably could STILL not pull it. Even that part is rigged. There again, it's a stunt for entertainment. Nothing more.