Terry's Jaguar Parts sells rebuilt diffs. They are out of Benton Il. You can also get new Dana 44 Gears ( that is what a Jag IRS rear is ) from various SPicer distributers. You can switch out the ring and pinions of the Dana with those from the (84-88)Vettes. You will have to change the pinion bearing, flange and make bolt spacers for the ring gear bolts since the new bolts are smaller on the OD than the originals. Tom's diff's in Paramont CA. makes the spacers.
I have a new .80 OD set for the Gforce 2.95 ratio 5 speed kits coming out in a few months. I don't like the low 3.27 1st in the TKO or the way they shift.
Some of my T5 / Gforce links are here:
Gforce T5 Stuff
Take care,
Paul