Get the "free" axle and keep it for when and if you need it.

Meantime  use a grinder with a cup brush to clear the rust

then give it a coupla good coats of cold galvanized spray paint,

available at Lowes err I mean Home Depot. 

Remember if you do replace the axle it will  have an up side that

must be observed . It should be marked on the unit.

 

Also those shackle bolts may be rusting too. If left alone

you may have  trouble getting them off to the point of destruction

and replacing them. Maybe put some lube on them for the winter.

Hope this helps

C22

 

Use this stuff carefully.  Get a drop on your finger and it will coat everything you touch.  We used to laugh and say you could paint the world with one can. 

 

Seriously, it is great stuff.  Used it to coat hauler assemblies, pump assemblies, etc., on my lobster boat.  That was a harsh salt water environment, particularly for the hauler which had salt water running over it constantly while hauling gear.

I have a Karavan under my Boston Whaler Outrage. it is galvanized steel. I have towed from one end of the country to the other and dunked it in saltwater many times. No rust to speak off. The original bias ply tires were junk, wore rapidly in a few hundred miles and threw their carcass off. I ran Marathon radials from Goodyear but they too come apart to often. Right now I have Towmasters radials. Superb tires. I have 15,000 miles on them and they look great and very little wear. 

 

Painted trailers unless they are galvanized under the paint just do not hold up well to salt exposure and even without salt exposure has been my experience that they rust badly after a few years. Aluminum or galvanized only way to go.