Do the hardened diffs come in kits for the whole diff, or just the gear
You have to buy the 38 tooth gear and the 13 tooth gear. They don't come together. The part numbera are: A856 for the 38 tooth and A857 for the 13 tooth. Not cheap but they are much better. I should have bought them in the first place, I spent more on the stock ones replacing them all the time.
As for the better chassis I don't know, I never ran the RS4 2. I wanted a shaft drive when I bought this one, it was a rtr in the begining, since I bought everything has been up graded. Now my son just needs to learn how to drive it.