Bean, your post triggered a memory. I suddenly remembered my Grandfather McDermott using the word in a similar way as you defined it ...To tug or strenuously pull up
I remember his tasking his sons (my uncles) to "muckle in a load of wood for the fire."
Haven't heard it since then. Perhaps it's an old-time phrase. Grandfather was born in Leeds England and mayhap he brought the phrase with him?
wow