The shims helped, but it still isn't working fully. For some reason, the throttle will only lock when I open it up wide (i.e. 65mph or so in 2nd gear). If I get up to 65 in 5th or 6th and lock it, the throttle just closes.
I don't think I'm going to have a working throttle lock in time for my road trip.
If I can find the time, I'll check Ace hardware and see what they have. I went there previously, looking for some kind of replacement spring.
Here are some pictures of the innards of this thing...