When you say 'spinning out' do you mean completely swapping ends, or veering off to one way or the other?
Completely swapping ends could be a diff problem, but is more likely simply a lack of rear grip. What surface are you running on and with which compound tire?
If by spinning out you mean veering off to the left or right, it's most definitely a diff problem. Try completely disassembling the diff, cleaning it with rubbing alcohol, and when you reassemble the diff, only lube the diff with a really light oil; something like bearing oil. I've found the type of lubrication used is much more critical than any amount of spacers etc.
The x-speed motor with only amplify any grip or diff problems you're having. Lastly, it could just you need a little more getting used to the car.
