you don't need to buy a slide hammer to get it out. especially if the axle is bolted in your car not just loose in a salvage yard. assuming you have removed the brake drum and removed the 4 self locking nuts, all you have to do is turn the brake drum around backwards, put two of the lug nuts on loosely and just use the brake drum like a slide hammer to whack it out. seriously they usually are not in that firmly and a couple whacks like that it will come right out.
but to echo veronica it is almost certainly a 28 spline. unless a previous owner has changed it out.
here is a nice website I found for decoding rear end tags.
http://www.fordification.com/rearends-ford06.htm