Both NC Hauler and Me again have given you the best advice. Unless you are seeing uneven tire wear, your axles are just fine. And your 15" tires are fine too. However, a move to LT tires will get you away from those chinese junk tires. The E rated LT's will handle your load.
If you are really convinced that you want to change axles, get your wallet out and don't go any higher than 6,000 pound job's. Then buy the 16" wheels and tires. But before you go there, check the wheel spacing to make sure they'll fit. Otherwise you'll need get a welder to relocate your spring hangars. And you'll need new axles aligned too ($300 or more).
IMHO, that whole thing total is a waste of money for your trailer. Just get 5 new LT tires made in anywhere but China, with a recent born on date, and be done with it.