STOP. Be very careful!
You need to pull out on the tabs pretty far. The problem is that with time, the tabs become very brittle and will snap off without warning.
You might consider heating the tabs slightly. Once you pull them out far enough, they'll release their grip on the body and the back will come off.
If you're successful, I strongly recommend you trim some plastic away from the two edges that lock on each other so that next time you don't have to pry so far. Good luck!