Lordy me, you must be loaded!
A new axle is lots of pennies, especially from Indy.
Looking at the age of it I would suspect that you have taper roller bearings. Also, I'd suggest that it would be worth checking the brakes. Nope, there's no way to sort the bearings, without stripping it down - so strip the hub off, do the bearings and check the brakes at the same time.
You can fit bearing savers, in the place of the (not OE...) dust cap. They are WELL worth the investment and are one of the few trailer running gear modifications that genuinely work. They're about £25 from memory. Get the OE ones, that are sprung, as they keep a continual pressure on the grease, pushing it in to the bearings.