Those rear discs seem to collect a lot of water into the axles, making them rust inside when not removed periodically. I once had a similar one here, had to sand off the rust and apply Liquid Wrench on the exposed inner portion, then cram it back in, then repeat, several times until I got it all the way out. Then I polished the whole length of it, and greased it during the reinstall so it won't re-occur.
