Did you try putting the loader bucket face down with a good deal of down pressure and then curling the bucket to use the bucket lip to flip you back while slowly backing up with diff lock engaged? Sometimes you only gain a few inches per attempt, but they add up until your tires can reach a better spot.
That technique has been very useful in my experience, but you may have been beyond it. It works best when executed as soon as you start spinning the tires.
As they say, 4wd doesn't mean you won't get stuck, it means when you get stuck, you're in a LOT worse place.