You need arm #2 or #3. #1 was for the first batch of 2.0's but they went back to the old style.

All the old diff parts fit. They changed the cut on the internal gears to make them smoother but the old ones still work if that's all u got.

Aluminum rear hubs and the axles fit. Also get the aluminum diff cross pins if u are going for lightweight. I heard they don't wear.