My money is on the rear diff...
Best advice is to sell both of the cars and invest in something a little more bash worthy. I can promise you that you'll blow through parts on these cars like there's no tomorrow and unless you've got a money tree in your back yard you'll be broke in no time.
If you want a 4wd buggy to bash with buy an 1/8th scale, you'll have more fun cause you won't be fixing broken parts every two min.