I too have noticed the shimmy you mention.
I just figured this was from wheels fighting for which direction to pull the car and I learnt to deal with it.
On carpet I found a diff much better.
On asphalt, I am running a spool and I am liking it more than the diff atm.
That being said, traction levels and track layout call for different setups. On a less technical carpet track I might try a spool. On a more technical asphalt track I might opt for a diff.
But in the end, I'll use whatever gives me the best results
-Mark