Unfortunately it's hard coded to 3 digits. You could write a script to rename your objects. Select the objects in the order you want them numbered, then run:for obj in selection do( obj.name = uniquename "Axe_" numDigits:2)
I think they recently bumped it from 2 digits to 3 in 2011? I had a handful of scripts that relied on the old 2 digit naming convention and they broken when they bumped it up to 3. Super lame its hard coded...