Actually you´re right,
I had a look and Tamiya really recommends the M05/M06 Ball Diff only with Universals!
If you want to go cheaper, M03 Ball diff (Manta ray) is just fine.
Mine is lasting very long now, built with AW grease and works still really smooth after probably 40 battery runs. Just tightened it 2 times slightly due to run in.
Unfortunately I just ruined the thread now somehow and need a new back plate.
So I will again give the 3 Racing oil gear diff another try...

In general I learned on this car, that the diff function is very much connected to the rest off the setup. Even rather small changes on the diff can have big influence on 180° corners both on turn in and traction out of corners.

BR,
Matthias