I didn't think Monkey Wrench was very "upbeat".
Maybe you're looking too much at the voice of the singer rather than the music that's being played with it? Kurt had a very somber sounding voice(pained, as I said), but Dave's is more....I dunno, goofier.
I think one of the best things about Foo Fighters is the way they've changed over the years, branching out and discovering new sounds and moods to encorporate into their songs. A lot of bands don't make transitions like that very successfully(Metallica didn't, and neither did some of Foo Fighters contemporaries, like Pearl Jam).
