Take it from my experience. The hellfire sux. The ss has some good points but many bad points.The hellfire pinions/crown are crap. They wear out very easy. The grub screws in the driveline falling out are a common problem resulting in no drive. The small pins in the cvd joints break easily as said above. The plastic diff cups are weak to and can result in your inner diff gears moving apart resulting in no drive.
I replaced my hf diffs with gst diffs which are heavy duty and never wear out even using modded os28s. You need to lower your clutch bell size 1 or 2 teeth if you do change to the gst diffs. The diffs fit inside the hf diff cups making an easy swap over. I got mine off ebay cheap. Best upgrade ever. There is a post in this section on the gst diffs if you search.
I found it very hard to keep my hf going. Always had something small go wrong with it. Goodluck and let us know if you sort out the issue.