For a first approximation, I'd start by removing the tyre/tube/rim strip, and flushing the whole freehub area with solvent. Do it outside, over a large tray and you'll catch most of it to run through again. Flip the wheel over and also flush from the back side. A squirty-bottle might help, as would a "meat syringe" Wear gloves and old clothes.
The idea here is to wash out all the lube, grit and dust. Manipulate the mechanism while flushing, and really get the solvent in there. Diesel can be a good choice, while petrol/gasoline is a little too volatile. You might have access to other solvents too.
The mechanical action should become less-gritty as you flush. Once it ceases improving, you're done. Deal with the catch-tray while the rim air dries. You might use plain compressed air to help that process, or an hour in the sun can do it.
Finally, you need to re-lube the mechanism internally. I would shove some wheel grease in whereever there's access, and possibly use a lighter oil like CRC556 or WD40 to wash the grease around. This is potentially quite messy. Another option is to use spray-on lubes. I suspect any chain lube would work well enough in this freehub too.
Let the wheel stand for a night with a cloth under to catch any drippings. If it feels good next day, reassemble and try it out.
If it feels rubbish still, you haven't damaged anything and can proceed with disassembly.
If the crunchiness is due to bearing balls being in the wrong place, this probably won't fix anything. Also if the balls have munched themselves into other shapes, this also won't help.
