count the teeth on your gears to see what you have. if you have the bulletproof design they will be 29t ring gear and 9t pinion gear. if you have a 43/13 tooth diff gears you have the early non bulletproof design, in wich case i would highly recommend you upgrade to the bulletproof ring and pinion. the non bulletproof ones wont last, a friend of mine went through 6 diffs in his flux conversion before we changed it to the bulletproof diffs. and yes, you have to change both at the same time because the gear ratios are different.