When I bought my 74 Boler
a couple of years ago, the original axle
was similarly worn. However, the previous owner had inserted riser bars between the frame and the axle to raise the ride because he had a big truck with a high hitch. We have a lower tow vehicle, so I removed the risers. I had the original bolt-on axle then.
The point is that you can get 1 X 1 or 1 X 2 square steel tubes, maybe a foot long or less, from a place like Princess Auto very cheap. If you have a bolt-on axle, you can drill 2 holes in each one and get a longer bolt to go through them to raise up your axle a bit. This is a way to get the trailer to ride higher off the ground without spending the $400 or so for a new axle.
If you do this, make sure the new bolts are high strength like the old ones, available at a standard nut and bolt shop (the strength number is stamped on the top of the bolts). Also, you should be careful to align the axle when you put it back on so that the wheels line up straight. If they are off alignment, your trailer will try to drag itself off line as you drive. This can result in a fishtail while driving. Miniscule adjustments can fix it, as both I and a friend who did the same thing have found.
Last year I replaced our old axle with a new one, also a bolt-on. The "ride" on the new axle is not really that much better than the old one. I think because the trailer is so light
, it still bounces around enough to rattle everything that is not secured, just like the old one did. If you buy a new axle solely to get a smoother ride, the result may not be totally satisfying. As an aside, my particular trailer seems quite a bit lighter than many fibreglass trailers, especially the newer ones with more and fancier (heavier) frame, furniture and accessories. Perhaps a new axle benefits a heavier trailer more than a light
Hope that helps,