tons of toe in and soft shoes on my Raleigh
From my perspective, this is probably the problem. Jawn P says to try setting the pads "heel in". My thinking is that harder brake pads are likely to help - they are less "sticky" and the force between pad and rim is stronger whenever you brake hard. (when the force between pad and rim is stronger, there should be more resistance to 'bouncing' motion or vibration). I also think that having your pads set with lots of "toe in" is problematic, and that you'll get less shudder if you try lots of heel in, a little heel in, or a little toe in. (see what seems to work best for you.)
The shudder comes from the long distance between the brake hanger and the brakes. Try to jimmy a hanger that fits right above the fork
With cantilever brakes, I don't think this is where the problem lies.