I'd guess that you've got either 3.73 or 3.55 in the stock 7.5" axle. 4.10 equipped axle choices are the Ranger 7.5" or upgrade to 28spline 8.8", or pop for an even bigger upgrade to a 95-05 Explorer 31spline 8.8 and get disc brakes as a bonus. Expo axle requires some work to bolt-up though.
Take a peek here for some info: http://therangerstation.com/tech_library/Axles.html
Check out www.car-part.com
as well for locating an axle. Or simply hit up your local yards with the info pulled from the link above....namely the pick-a-part type yards. Prices on axles are generally about $100 u-pull.