@worthlutz: Sorry, haven't got time to check this out for you, but maybe you can try this:
- copy the json to a static file on a server (something.json)
- point your proxy there
- see what happens if you remove elements that are not needed
This might not solve your problem directly (if you can't edit the service for example), but it might help the guys at Sencha figure this out if you can pinpoint what's causing the problem.