[LMH]

Like Bob says, the expensive part on a 31 spline IRS diff is the stub axle. It took over a year to find mine as I didn't want to pay for them from the dealer. The Driveshaft Shop did my axle shafts.
On an 8.8" IRS unit, one of the other weak links is the rear cover and it's mount. I've seen the cover broken a couple times. I have an after-market cover that's much beefier.
Larry