Why does my rear diff keep undoing itself?

[Cassp0nk]

Finally had a chance to take my car on its first test and after only 2 mins of running the rear diff undoes itself?

Is there something specific I could have done wrong in assembly that could cause it to undo so quickly? The grub screw is done up tight and the front diff doesn't do this, although it does feel a bit notchy in comparison.