The stock diff can work well. you can build it per the instructions, but use a little anti-wear grease to tighten it up a bit. If you go with the M-03, you will find diffs made for the M-05 won't fit, so use a TA-03 ball diff. The best upgrade there would be to substitute hardened or ceramic diff balls as the stock ones are kind of crappy. A little anti-wear grease, crank it down and you've got traction.