[starscream]

I have experienced the same lurching after a fast run down the straight...
neither of the diffs should have any affect with coasting though so I have found it difficult to identify as well.

The diff's should mostly only affect on power handling. Any lurching would seem to indicate either binding somewhere in the drive line or the esc/motor braking. I used a metal pinion and it was eaten up after just two runs (Metal to Metal). I just picked up some hardened steel pinions today, from Mega Motors, so I'll let you know how the hold up and if the lurching issue is resolved. I am also working on a very simple plastic spur solution so I'll report on that as well.