It's short for "shuffling" which has a couple of meanings. It usually means moving ones feet in a clumsy manner. In this context, it probably means dancing with a shuffle, or moving in a clumsy manner in general.

"Shufflin'" is a slang, shortened version of the word "shuffling," the present continuous tense of the verb "shuffle." In normal use, "shuffle" means to move your feet slowly and without lifting them off the ground; it can also be used to describe the mixing of playing cards - i.e. "shuffle" a deck. So, "shufflin'" is a slang term for a type of dance where the performer moves his feet slowly and without moving them far off the ground. There was a dance in the mid-80's (I think) called the Ickey Shuffle, named after a wide receiver for the Cincinnati Bengals that did a dance after scoring a touchdown.

"Shuffling" usually refers to dancing. In particular, the shuffle is a specific dance that sort of looks like the people doing it have some sort of nervous disorder.