[RRich]

Check out the ones from Nth Degree. They protect the cover and extend under all the way to the other side, protecting the U-joints too. Both front and rear.

I made mine, then saw they had a similar thing.

 

[RRich]

The two sensitive areas are the cover and the Yoke. A well placed rock can hurt either.

Here's mine:
Front diff:

 

[RRich]

Under the yoke - yes, homemade, but it works. You can see how beat up it is.

 

[RRich]

And the rear diff. Actually they just sort of grew - I started out with just the cover, then added more underneath, then added more to that. I've made a few more since - they look better, but are essentially the same. The main backbone is 1/2" thick.

These are well used as you can see. Imagine what the stock cover would look like, and how many times I would have bent up the driveshaft yokes.

When I hear a crash or crunch underneath, I'd rather laugh than cringe.

 

[RRich]

Yup, hard stuff alright - till it shatters. Seen 2 like that, not pretty. Hopefully the skid distributes the impact over enough area to help prevent that. Plus there's a slight bit of "springy" to it too.
It's probably overkill for what I do with it, but that's better than underprotection.

Notice the rails on either side of the driveshaft - that helps prevent getting hung up on the big open area behind the tranny and ahead or the rear diff. Also well used.