To make a good suggestion, we HAVE to know more. So what are you breaking in the rear end? What kind of power and torque is the engine making? Is this an overheating problem, or a matter of not having lubrication where it needs to be in the hard corners?
It is common for drag racers to break axles and that is a simple matter of going to 31 spline axles with a matching limited slip differential. From what I've seen, an 8.8 with 31 spline axles and limited slip diff are a nice setup that can take LOTS of abuse. But this says nothing about long term durability and limited slip isn't ideal at tracks with corners. Back to road racing, what kind of differential are you running? From what I've read, most of the differentials that are good for road racing simply don't have the strength of the standard limited slip differential.
I recommend reading mustang performance handbook 2. While the book is older, it has a lot of great information on modifying the mustang to go road racing. Here is a brief summary of their suggestions: Add an extra vent on the left side axle tube. This allows off gases from the grease which is breaking down to escape instead of blowing seals. *my opinion... If the grease is breaking down that quickly, you should probably change it before every event, which can be easier if you buy a differential cover with a drain plug* The book also recommends one of 2 setups... installing 9" ford axle housings on the 8.8 rear end or purchase the Moser kit. The Moser kit would supply 2 stronger axles and 2 bearing carriers that are from a 9" ford housing. They state that this axle/carrier bearing has proven itself in millions of miles of racing, so it might just be your solution. I'm far from an expert on the road race stuff, but hopefully that gives you some ideas of what to research.
What about your competitors? What kind of setups do they run successfully?
I just blew my third rear end in the last six months. 8.8", 3.90 gears, that's all I'm going to tell you guys for now.