Cougar 2 diff problem?

[McMike]

Just been examining the rear wheel spin on my recently rebuilt Cougar 2 and think I may have spotted a problem. I have the team racing model so it has the slipper clutch on the spur gear if that helps anyone who may be able to help me.
Everything is hooked up and just waiting for my batteries to arrive and, for no reason, I decided to just spin the spur gear. It was then that I noticed the left rear wheel was spinning more than the right rear. The diff itself has nothing missing and was completely rebuilt so it operates very smoothly as a diff it just seems to have a problem now that it is inside the gearbox and is attached to the belt pulley. I don't know why the diff would spin the left rear quicker than the right in both forward and reverse.
Any ideas? Other than taking the diff apart again and rebuilding it I can't think why it would do this. I always had a problem with diffs anyway!
Cheers.