Part of it is going to depend on how tight your diff is.. if you've got it relatively wrenched down its going to want to spin the motor before it actuates the diff itself. If you hold the spur, is it the same way?
Does it feel 'crunchy' at all? Its possible you've flat spotted the balls in the diff, which will cause what you're seeing - the flat spot basically causes a bind.
I've been using the BFast diff rebuild kits in my Losi for a while now and I've been quite happy with their performance (much smoother) so you may want to give them a shot as well. Just make sure you break it in properly.
Tekno e-D8 (build in progress)
Kyosho Lazer ZX5 SP