Which words make up the prepositional phrase in this sentence? along the back fence shuffled what looked like a large gray cat.

Along the back fence is the prepositional phrase because prep phrases can't have verbs; only clauses. Prep phrases have to include a preposition, noun, and usually an article (a, the, an).

Here's a song so you can remember prep phrases better: https://www.youtube.com/watch?v=yfExXGMX2JM