Pulled this info off another Sprinter forum: Sprinter - Sprinter-Forum
"While two rear sway bars does a great job controlling the vehicle from side winds or a truck passing, it has a negative effect when crossing dips or speed bumps at an angle. You will get a lot more side to side rocking.
Ideally, you want independent rear suspension. In other words, when you hit a bump with the right rear tire, it shouldn't energize the left rear suspension. But it does. The sway bar couples the suspension. In other words, it provides positive feedback to the other wheel. The stiffer the bar, the greater the coupling. As the right wheel suspension compresses (from going over a bump), the left wheel suspension expands. You're now set up for lots of rocking because the sway bar continues to provide feedback long after you passed the bump.
The resonance frequency of the suspension is about 1 Hz. That means if the second rear wheel hits the speed bump a half second later, you're energizing the suspension at its resonance. You can expect extreme rocking.
There's tradeoffs when considering the stiffness of a sway bar. There is a reason no production vehicle has 2 sway bars"