For backpacking I think a Glock 29 would serve you pretty well. You can find some pretty hefty 10mm hunting rounds out there and having 10 shots with a semi-auto is a pretty substantial defense against any critter. Besides, it's a Glock ;f
Failing that, I would think a Ruger in .44 mag with a 4" or 5" barrel would be nice, harder hitting, but with less rounds and I would imagine a good bit more weight (evil thing when backpacking).
"Speak softly and carry a big stick"