The axles themselves don't need to be done in pairs but if your changing the gear ratio, the front and back need to match.
Unless your planning on changing the axle for something like rock crawling, the OEM one is fine just about anything you'll throw at it. Is your current axle broken? If it's just leaking that can be fixed/rebuilt. You could check the local salvage yards (
www.car-part.com ) for a low mileage, that axle has been in use on the HD trucks for I think the entire Gen4 body style.