While I was looking for drive shafts to fit the Frankendiff, I found these.
Drive shafts.
They come with 2mm, 3mm, 4mm, and 5mm bores - and in a variety of lengths.
I did notice that the manufacturers are not always careful about getting both ends of the drive shaft properly clocked. On a full-sized vehicle the knuckles being out of phase will cause vibration. Will that really matter on a model... I dunno.