Those washboards can be so severe they just bounce the vehicle sideways. Glad you both are OK. Thank the modern safety technology in that truck. Some people are naive thinking it couldn't happen to them.
Agreed. I've never gotten into a vehicle and not put my seat belt on for that reason. I had never been in a wreck before last night but I've had plenty of friends who were and I know it can happen in the blink of an eye, just like last night. I was incredibly impressed with the truck and how well the cab held up. The front of the truck was crumpled and obviously the back was destroyed, but the cab held up. Between the seat belt and the airbags, they both did a ton to keep us from being hurt any worse than we were.
40mph could be considered an insane speed depending on road conditions.
In the end yall learned a good lesson and survived it. 
The road was great up untill that corner. There's a big gas plant out there so they keep the road in pretty good condition. The corner was a wash board though and it was to late to do anything by the time we realized the road had suddenly gone to crap.