Blinkin eck, that's something you don't want happenning too often. When I first started building my new monster I was very concerned about mounting the diff. The forces involved in an IRS set up are very complex. The assembly has to be able to handle the torque from the engine, the torque driving the wheels and the lateral forces involved in accelaration and retardation. It basically seems like every part experiences forces in every direction under one condition or another. Any weakness in any direction can result in failure. There is a web site in the US of A dedicated to irs setups its this one here:
http://irsforum.boardhost.com/viewforum.php?id=1 There may be some useful info buried within that will help get to the bottom of this.
Best of luck.
Dave.