I had a set on an early maxx, and while they really helped on a flat track, they were terrible if you wanted to jump with them. I didn't keep them on long.
Full Force RC made something if I remember right. Can't recall how they connected though... but they were a solid pivot, not really a spring/wire sway bar like you normally see.
It was anti-roll bar though, no flex to it at all. If you raised the right wheel, the left would be mechanically linked and raise with it. Downloaded a photo of it in the package, you can kind of see it in the instructions inside the package: