"Rock" is not much better than "ruck." If the point of singing the song is to express a mother's grief and worry, then it's no good setting the listeners to wondering why she would sell a rock. Even if a rock is part of a spindle, no one's going to know that.

The deeper meaning of the verse is that she has sold off the things she needs to make her living in order to protect the Daddy who has left them. In my opinion, it's more important to convey that than to cling to one archaic or dubious word.

I think this song has gone through much revision over the years, and this is just another revision. For example, the line "gone the rainbow, gone the dove" sounds modern to me. I like it, but it still sounds modern.