After looking around the internet trying to see suspension upgrade for our RWD with 4 link and panhard rod, i found this ( watts linkage ) which is used on a lot of car with rear 4 link like Mustang . Our car, AE86 , the panhard rod will push the axle toward right side when the rear of the car go down during high rpm launching, when the road surface suddenly lower/ not even ( absorber contract), this pushing to right side will made the car to be unstable. The normal way to solve this problem will be to set your rear suspension a bit hard to stop the car rear to go down thus stop the pushing of the axles to right side by the panhard rod
Here are the few picture of the linkage, for more detail/ explanation search the web for the key word ( watts linkage)
Here are the few picture of the linkage, for more detail/ explanation search the web for the key word ( watts linkage)