There would have been an 8 1/4 SURE GRIP, usually mainly found in wagons and some 360 cars and better equipped coupes More commonly found in J bodies and M body police and taxis.
That's the direct bolt in. There are other options that are easier to find.
It's actually 66-70 B body. I ran one before going D60 and it was pulled out of a '66.
In all honesty, this stuff is getting real hard to find now. Unless you're looking to keep it Mopar, a Ford 8.8 from a 90's Mustang is way easier to come by.
Not sure if you are considering having a sure grip installed in your 8 1/4 original rear or you are wanting to change the axle out completely. A original sure grip in a 76 would have been a cone type unit. They do wear out over time and are considered not serviceable but some have rebuilt them with machine work and shims. The spicer unit would be a better option IMO. Mopar 8 1/4" Spicer "Trac-Lok" Differential
I have upgraded a few FMJ's, back in the '80's, to Limited slip. The new vehicles came with cone type carriers, but the clutch type was still available over the counter - but were a bit more expensive (maybe $150 vs. $140 for cone type - or something to that effect) for the part.
BudW
I need a rear axle housing to fit into a 1989 5th Ave. Obviously Diplomat/Gran Fury/Aspen/Volare (1980-1989) should work. The ratio needs to be 2.94 to a 3.21. Mine currently is 2.71. Even though this is an "M" body I have used an Aspen rear axle assembly before and it bolted right in. Thanks.