Sure enough! I was right, so I dumped the contents in the sink and rinsed the carton out to prep for recycling. Then I ran the disposal.
The ungodly racket that burst forth from the dark realms of the sink drain was deafening. I switched it off, ran the water for a bit, then poked into the disposal with a knife. There was something down there.
I stuck my hand down into the sink, and the disposal turned on of its own accord, destroying my hand... Okay, that didn't happen. Instead, I pulled out a rather large lump of glass. My first thought was "Oh my god! Some jerk-ass at the sour cream factory put a lump of sharp glass in the bottom of this carton!"
But I reached down again and found several smaller glass shards, one of which read "Salt La-".
But now, the disposal no longer runs... Great!