Cosmetic surface rust should not be an issue. When you say "drum" I assume that the unit has brakes (unless you mean the wheel, or hub). Assuming that the brakes are used regularly and the friction surface is not flaking with rust, these, too, are probably okay. A good clean up with a wire brush and rust inhibiting paint should sooth your eye.
If you have large areas with heavy flaking rust, or deep pits, any holes rusted thru, etc. then it is time to start swapping to new parts.
Post a few good pictures and we can help you better assess the situation.
Unless the thing was located near saltwater, or hauled on salted winter roads, I would not expect an axle that new to have any major problems, outside of the normal bearing service (or lack thereof). In fact, once upon a time I bought several surplus Dexter leaf spring axles that had been laying outside uncovered for years of New England winters; a few of them still are and there isn't enough rust on them to come anywhere near condemning them. When I used a pair of them on my tandem flat bed, all I did was hit the insides of the brake drums with a green abrasive pad, quickie wire wheel, slapped some POR15 on the outsides, and they where good to go.
