To create alieron differential on a single servo you need a wheel instead of a dual arm. The holes for the linkages are made foward of the center of the wheel. I can't think exactally how to draw this.... Instead of making the holds directly across from each other and across from teh center of the wheel, the holes are made across from each other and foward of the center of the wheel.

So instead of this:

0 X 0


Where the 0's are the linkage holes and teh 'x' is the center of the wheel,

You get this:


0 0
X

This gives you differential travel