Sorry I'm late to the party. I had a similar issue with my dog getting tossed around like a ragdoll on (seemingly) moderate trails.
I tied a rope between the two rear "oh shit" handles, and then clipped a leash to that in a way that the leash could slide either way on the rope. I hooked up the leash to the dog's harness and he's now way more comfortable on bumpy or off-camber rads.
Not sure if this is what you're looking for, but hopefully it can help as a jumping-off point.