can you tell an axles gear ratio from a serial number on the axle?

Yep...it's stamped into the forward side of the passenger side axle tube...you can reference the code here..http://www.hioutput.com/tech/ratio.html

Pete

it's usually easier to just pull the rear cover and read the ratio number off the ring gear

a lot of times they're stamped so lightly or covered by crap you can't see them anymore...