The "underslung" axle came with the trailer, but they are available.
Reliable UNDERSLUNG 4in. Drop Axle | Axle Kits | Northern Tool + Equipment
and other places. You probably should get trailer brakes while you're at it.
I was going to attach a picture, but I can't figger out the forum protocols. Anyway, I have to thank you
-- When I was measuring my keel, I realized that it was sitting on the axle. I had thought that was OK, then I realized that every time the trailer hit a bump, the keel would be pushed up into the boat! Not good at all!! I'll have to come up with a new support system thet holds the keel up above the axle a couple of inches.