What do you mean a crawl-mode..a multi-mode transfer case? That's the last feature BMW would consider (SAV not SUV); they actually never would. Even if so, I don't understand how that'd make going over a curb any smoother. Your smoothness issue is more likely due to the dead spot in the intial tip-in of the X5's throttle. And it also probably has to do with the X5's firm suspension, you didn't have a hop/bounce because the RR was more compliant.