I'd be a little surprised if it's a Dana 70. I had an early '85 F-250HD, with a GVWR of 8600. That had a Dana 60 full-float.
Dana did usually cast a number in the housing, usually in very large font (letters a couple inches tall). That should confirm if it's a 70 or a 60. but sorry, I don't know how to narrow it down more than that.
Bob
Sorry, no '80 - '86 Ford trucks
"Oswald": 1997 F-250HD crew cab short box, 460, E4OD, 4.10 gears
"Pluto": 1971 Bronco, 302, NV3550 5 speed, Atlas 4.3:1 transfer case, 33" tires
"the motorhome": 2015 E-450-based 28' class C motorhome, 6.8L V-10
"the Dodge": 2007 Dodge 2500, 6.7L Cummins