Advice needed vs 71 Duster rear wheel bearing replacement

Just LOOK at the axle. AND DOWNLOAD THAT MANUAL 8 3/4 has a 3rd member, you don't have a Dana 60 so the only other two possibilities are a 7 1/4 or 8 1/4. 8 1/4 has an "oval" cover with flat top and bottom, a 7 1/4 has a sort of "laid over pyramid" shape, see this

You won't have a 9 1/4 unless someone built one and swapped it in there

Mopar_rear_id.jpg