diff question

[sldmodels]

my micro's have never really had enough run time for me to think about rebuilind the diffs, but when I was testing with the car on saturday (on regular car park tarmac), the car was pulling to the left. I had a little play with the diff, and compared to my tc, it could be a bit better, but was it more likely to be pulling because of the surface, or the diff needing looking at?

The reason I'm thinking the diff, is I've burnt out 3 motors, and possibly 2 speedos since before the last Dudley micro's I was at.