Duster rear end compatibility

If you're on a budget, I would look into a Ford Explorer 8.8 swap. Cheap at the pull-a-part, limited slip, variety of ratio's, and easy to narrow to A-body width. Write-up's are available here on the board.
Unless you are a purist and don't mind paying the price for an A-body 8 3/4 that may or may not have the ratio you want.