Just to muddy the waters, "to shift" sometimes was used to describe someone who did "shift work," i.e. one who showed up at the union hall and worked a shift if there was a job to do, or just sat around until time to go home if there wasn't.

I've heard: "How you been, Joe?" "Well I shifted for 6 months but I've been on regular for the last couple."

I don't know that the term was ever used regularly except on the coastal docks, but I've heard it from "lumpers"* with the moving van companies elsewhere.

* "lumpers" are the load/unload crews, usually hired locally by the driver, to load or unload furniture or other cargo. Most common in the furniture/home moving business, since other kinds of stuff usually loads or unloads at a dock where there are regular workers to do it.

(And other truckers call the furniture movers, drivers included, the "bedbuggers.")

I wouldn't think this usage would be applicable to the song, but then I don't know much about the weaving trade.

John