How do I find an 8 3/4 rear in the jy

You need to go back and check the center section. It might have a 3.55 in it or if your really lucky it might have a suregrip in it.
Shhh, you’re giving away my secret of finding Suregrip 8 3/4” 3:55’s and 4:10’s in old 4WD Dodge trucks from the mid 70’s. Next thing you know you’ll be telling them to check the C bodies too, especially the 68-70’s, for those 742 case Suregrip’s with 2:94’s that go for cheap.