Oh, those axles are terrible. No bearings... wow. Those are just going to grind up any wheels you throw on there, and they will just get sloppy again like you have seen already.
I am betteing those are just a couple short pieces of all-thead theaded into each end of the 3d printed pos axle tube. I bet it doesn't go all the way through.
I bet those springs used to be arched the other way too. They weren't designed properly.
Sorry to say, but if it were me, I would keep just the aluminum deck and scrap everything else. You're wanting to haul heavy stuff with that trailer, and by the time you fix everything wrong with it, you could have built a new one with angle iron and c-channel.
Fabbing up a new axle housing to utilize bearings will be the hardest part.
Here is how I would do it, but it would require some fab work, welding, and some machining.
Rectangular tube with a short section of round tubing welded under it. The length of the round tube would need to be determined by buying a pair of 2wd buggy front axles, bearings, bushings, etc. Then both ends of the round tubing would need to be machined to accept bearings. You would want the length of the tube to be just so, so the screw in the end of the axle clamps the two bushings on the inner races of the breaings. Hard to explain, but maybe this will help. Basically, if you slid both bushings onto the axle, then measured how much of the axle was sticking out, the tube needs to be just shorter than that.
You can buy metal leaf springs in 1/10 scale from various shops. RC4WD makes some.
Other than that, I can't really help ya much as it's all design and fabrication, which will all depend on your skills, and what components you have available. It's a tough project to take on if you don't have the skills. But you can learn. It just takes trial and error.
Good luck.
Oh, and here is how leaf springs should work. And is a similar axle design utilizing flange bearings pressed into the 3d printed axle tube.
