1968 Dart sudden rear end noise.

If your existing differential is not repairable, or too costly to fix, a b body 8.75 or 8.25, or a Ford 8.8 from a Ranger pickup, or an Explorer are also options, with varying costs involved. The Ranger will have 28 spline axles and drum brakes, and the Explorer will have disc brakes, 31 spline axles, a Trac lok differential and either 3.73 or 4.10 gears. Both Ford differentials will also have 5 on 4.5 bolt circles.
Best suggestion I can make is to first find out if you can repair what you have, then do a whole lot of research on the subject. There are threads on the subject here on the forum.