I was watching it on DVD today, and at the 20:23 I heard Goober say "Yo." Did anyone else hear it too, or was it just me?

You are correct. Goober does say, "Yo" in that episode. By the way, "Fun
Girls" is the only episode of the series that features both Gomer and Goober.
However, they did appear together two other times: once when George
Lindsey guest starred on an episode of Gomer Pyle, U.S.M.C. and in
the made-for-TV movie "Return to Mayberry."

He did indeed say "Yo"--first time I ever heard the expression in fact. Gomer translates ("That means 'Yes.'") Goob also says it in the reunion movie, affirming he'll get the carburetor Andy needs.

I was watching it on DVD today, and at the 20:23 I heard Goober say "Yo." Did anyone else hear it too, or was it just me?

You are correct. Goober does say, "Yo" in that episode. By the way, "Fun
Girls" is the only episode of the series that features both Gomer and Goober.
However, they did appear together two other times: once when George
Lindsey guest starred on an episode of Gomer Pyle, U.S.M.C. and in
the made-for-TV movie "Return to Mayberry."

Great episode, I watched it last night. My favorite is the Pickle story. The looks on the actors faces when they eat the kerosene cucumbers is priceless. I bought the 50th anniversary dvd for 15.00 at Wal Mart. I also enjoyed the commercials that are included in some episodes.