Since it tries to turn the wheels in relation to the axle, and not the frame,
hydro-assist does decrease the need for panhard-bar somewhat,
(It did for me anyway)
But some-times thats not enough...
I have really soft 56" leaves, with pretty long shackles, and stock rubber bushings in the leaf-ends.
I have considered a panhard bar, because it isn't funny at all to drive it on the road.
But I will try poly-bushings for the leafs first, since the rubber-bushings seems to be the worse culprit.
(they are not designed to take upp side-way forces, and flexes quite a bit)
An easily removable panhard-bar could also be an option...
Cause when you're offroad it's really not a problem