you would need to have the diff apart to see the spring, with the drive shafts out, you should have the head of the bolt one side and the retained nut the other, should be possible to adjust the diff by turning the bolt with a hex driver and locking the other side of the diff, unless I am totaly misunderstanding you, do you have access to the manual. (I am basing this the original kit, but would expect the rtr to be the same in these respects)
edit, just spotted Simon's post, think there is a link to the manual from jimmies original review in that section
http://www.oople.com/rc/manuals/zx5/