Redline is good stuff. Don't know about the shockproof though. Here's a case in point. My pop's old race car once ran 3 60 mile laps with no gear oil in the transaxle, when the trans guy opened it up all parts had almost no wear on them.
Well I spoke to Redline yesterday - they said its not really for use in a bronco rear diff and that I'd be better of using their standard 75w90. I currently use castrol but may give the royal purple a shot.
I use the Royal Purple as well and it seems to do a good job. Just remember to add the friction modifier if you have a limited slip differential even though the Royal Purple says it is designed for use with a limited slip. Mine still needed 4 ounces of the stuff to keep it from locking up in a slow speed turn.