spookypt,
I'm going through this on a 2003 Mustang Cobra IRS in a (drum roll) 1981 Mercury Zephyr station wagon with a 557 that is intended for open track use.
After reading a bunch about problems on the track with overheating I concluded it needs an
oil cooler which I'm in the process of installing. At the same time, I've installed the newly available Ford differential cover:
1999-2004 COBRA IRS AXLE GIRDLE COVER | Part Details for M-4033-G3 | Ford Racing Performance Parts
It has two major upgrades: first, it's waaaay stronger and has diff bearing support caps, and not surprisingly it has all the holes needed to add an
oil cooler.
Here's a sample of how some correct the problem (I think this one is pre-new-cover):
IRS Differential Cooler:
In your shoes, I might try a cooler first, then if another failure occurs, go to the Ford IRS diff. Don't scrimp on the size of the cooler.
One theory is that the straight axle holds more fluid and has much greater cooling area if you include the axle tubes.
The IRS is kind of shrouded and has only the flexy aluminum diff to get rid of heat. The track guys say the least that happens is that the diff fluid boils and then the limited slip gets cooked. Don't know what might happen after that...
My Cobra has a Jag rear, a very large cast iron chunk with inboard disk brakes and has survived many 30 minute track sessions just fine. Newer is not always better!
Just stuff to think about,
Tom