I'm no expert, but the folks on the U-Haul
Facebook site say pretty emphatically that they are NOT serviceable. But apparently also give very long life just the same. There is information on that site on alternative hubs that can be substituted, should yours go bad... or if you wanted a spare along on a long trip??
Definitely worth joining both here and "over there". Different format between the two sites, for sure, but lots of help to be had from both.