RE: TT-01 Rally
I would recommend the square racing diff locker for the rear when running rally or drifting, with a stock front diff it allows you to pitch the car and drive it more aggressively IMO(both front and rear locked is a ball when drifting). 3 racing has now come out with a solid axle for the tt 01 and its like $5 cheaper but it uses its own special ring gear, I would prefer to have a solid chunk of aluminum in the stock case with the stock gear like the square racing one. Hobby lobby sells some .010 lexan which you can melt with a heat gun or a very good hairdryer to help seal it up.