I'm looking for a module that will show how much more the customer has to spend to get free shipping based on table rates. eg frees shipping after $50 and cart has $35, it will show 'Spend $15 more for free shipping'.
Does this exist or will it have to be a custom job?
This sounds like it will do what you want;
For this you need to develope a custom extension that provide this functionality.
We provide magento customize service. If you want to make this feature for your website, you can contact us on the link: http://www.magewoo.com/magento-services.html or send email via email@example.com.
Hope it's helpful for you.