Based on the examples I've seen you're doing it wrong.
onDrop
callback should be declared injqyoui-droppable
jqyoui-droppable = "{..., onDrop: 'myCallback', ...}"
But look @ this for a more complete code here
Solution
Lil bug though, it drops the first 2 items and nothing more but this should help you on your way. Update: I just noticed you had a limit placed on it, silly me. In that case, it's solved