As hard as you hit and the direction of forces at play there is every possibility the the subframe itself has been tweaked hard. Might be more cost effective to hit the boneyard and grab a complete rear suspension, you can probably leave the differential and the axles if that would bring the price lower. The set of independent links for one side will end up costing almost a couple hundred and might not fix the problem. Even a minor tweek in that suspension geometry will cause crazy problems at speed that will be difficult to diagnose exactly by anyone but a dealer ultimately.