How do you hold your slipperential while tuning it.

[Dafni]

I don't hold the shafts, I just press the whole truck down so the wheels wont spin. Then you can turn the nut a bit, move the truck a bit, and give the nut another bit of a turn.
Or when it's already pretty tight, you can grab the spur itself. Works up to a point, when it starts to slip when you turn the nut.

Hope that helps

Daf