The GM 8.5" rear was available with 2.56 and 2.41 gears. It was used in 1/2 ton trucks starting in 1980. To get 6 lug shafts you will need to get one from a 4WD truck or else swap the shafts from a 4WD truck into a 2WD housing. If you get only the gears and want to put them into a different rear which originally had 2.73 or shorter gears, you must also get the differential case (the part the ring gear bolts to). The case you need was used with 2.41 and 2.56 only. Parts availabilty and aftermarket support for this rear are extremely good.
http://www.globaldrivetrainsupply.co...ferential.html
I haven't got a clue about the Jaguar rear.
Ray