I also bought a complete set of axles from Inland. Buying there simplifies the whole process quite a bit since they can tell you exactly what axles you need based on your trailer age, size and serial number. There is also a plate on the axle cross member that gives some data about the axle. That is it will if it is still readable. I could barely make out the data on my old 1973 axles.
If you go to the Inland RV web site there is a qustionaire about your axles that gives you an idea of the basic information that Inland needs to send you the right units.
You might also want to check the various threads here in the forums for more information. Check out the threads titled:
Need New Axles
Stupid axle question
You can search for these titles in the "Search" menu at the top. You are likely to be able to find some other postings about axles too if you search for the key word axles.
Hope this helps some. If you have other questions please feel free to post them as there are plenty of answers here in the forums. I have learned a lot of things that way.
Also be forwarned - a new set of axles is not cheap.