Welcome to the on road scene, there are no rules concerning diffs, spools etc in most classes I think except oval racing.
Anyway if you are getting an Xray look at the set up sheets for the team drivers,
http://forum.teamxray.com/viewforum.php?f=8
can't remember the last time I saw a front diff, most people who do run diffs use putty in which gives an almost spool feel.
Personally I haven't used one since the T2 009 and didn't like it in that.
The only exception to the above is if using foam tyres (a spool will increase wear in all drivetrain components due to the extra grip foam tyres can give)