Yay, changing diff wasn't too hard and I even got everything back together again nicely.
And then I spotted a spacer on the table
So I disassembled it again and this time did it right.

A thing that really annoys me is when hopup parts don't come with everything needed to install it.
The yeah racing steering assembly manual states "insert spacers here (*not included)".
The 3racing gear diff needs M10x15x4 bearings (*not included).
If there's anything that kills the joy of working on your car it's not being able to make it work because of missing parts.