If it's actually a threaded axle it's on a full floater, 3/4 ton & up. Whether it's that or a front spindle, it's easier to replace than to rethread. On a spindle, you're trying to weld high strength steel, which will have to be heat treated. If it's a rear axle housing, you'll have to go to a specialist, welding the cast steel will be easy, finding someone with the tools to rethread will be tough.
