Splines don't crack the cover. Shrapneling the spider gears does. As the broken pieces migrate around the ring gear shoves them thru the cover. And/or the broken pieces went thru the ring gear and pinion, causing the bearing cap to shatter and push the rear cover out. (crack).
I'm guessing you'll be needing an entire diff assembly. It doesn't sound like too much will be salvageable inside yours. But get it out and open and report back. Take pics too.
Also see this thread.
http://www.viperalley.com/forum/vipe...-quaife-2.html