I guarantee you it is a setup issue, default setup for the RWD cars are horrendous, a little tip to push you in the right direction, you want softer rear springs than front, the car need to be able to shift the weight to the rear for grip, so it needs go be quite soft, and fiddle with the diffs, if its off throttle you're struggling, adjust preload, if its on throttle adjust diff locking % cant remember the exact name of the setting of the top of my head and i am not infront of my computer right now, but hopefully you get which setting i mean. I have WR's in RWD grp b, so think i am doing something right, you also can't just stay full throttle all the time, especially over bumps and dips, RWD cars require a lot more and finer throttle control than say a modern wrc car, or any AWD car really.
I dont wanna share my setup, as i did after all put a lot of time and work into it, but if you need any more pointers i can try to help you out. Good luck 😊