Are double locker pretty common out there? I used to work as a grease monkey in a Kenworth dealership doing mostly transmissions and rears and we hardly ever saw double lockers except for on a few dump trucks and cement mixers.
They're pretty common. We get into some steep ground and they really pay off. The only bad part is that you can't get around a sharp switchback with everything locked in. The drivers get really good at quickly engaging and disengaging the lockers. You don't want to use them at highway speeds either...makes more work for guys like you.