As of v0.34, TweetDeck allows you to add a custom URL shortening service.

You are able to enter a URL shortening endpoint by navigating to: Settings > Services.

Then, click on the URL Shortening drop down box and select: 'Other'.

You will then be presented with new options which will allow you to enter the shortening endpoint as seen below.

Enter the endpoint of your choice (some examples provided below) and then click 'Save Settings'.

From now on, any URL you enter that needs shortening, will be shortened using your custom service.

Sample List of URL Shortener Endpoints