Just bought this one from rcmart, if you're on brushless motor then you MUST use this alloy transmission joint because your stock plastic join on MB4 surely ripped out to shreds due to excessive high torque load especially when you have alloy/steel universal drive shaft. The alloy joint is very thick and surely would able to handle direct torque twist from differential.
These are necessary if You're going to upgrade to brushless power! I've tried both the stock differentials and ball diffs. I think the original differentials are great and they can, except for these, take a lot of punishment!
I suspect they will get worn a bit quicker then the ones in steel for ball diffs, but it'll still take quite some time.
Thay could've been a bit cheaper, but they're still alot cheaper then ball differentials.
If You want smooth diffs and a car that can take all that brushless power, this is the way to go!