dbooker630;
We use UT for flaw sizing and detection in welds and base metal on various pieces of equipment at our Plants. One word of caution, you should have a qualified NDT inspector perform this activity, and the technique must be verified so that it can be repeated with confidence in production or even in the field.
I can't provide any additional information about your specific application other than contact a local NDT firm and have them evaluate your application. I will say that you might end up having to destructively examine one of the axle welds to provide a suitable mock-up and standard for UT application.