I changed the axles on a previous 5th wheel, but I only did so because I was significantly over-loading the OE axles (which were undersized based on the GVWR). It can be done, but it is a lot of work. And these things have a significant number of key measurements; they need to be manufactured to fit your trailer, spring hangers, width, drop, etc. You can work with Dexter to get all the measurements they need, but it can be a fairly large project.
I would be reluctant to replace the axles because they are "making terrible noises" - the likelyhood of this being the axles is quite small in my mind. As George and others have suggested, I'd start with weighing the trailer and making sure you are not over loaded. Then inspect the wheel bearings and races to see where the noise is coming from. That also allows you to inspect the brakes, drums, and spindles. My guess is that if you have a problem, it won't be the axles. Also, as previously stated, trailer axles are under a lot of stress when making a sharp turn - they do make "strange" noises until you get used to them. JMHO