Same as any rear end, you need to have the correct side gears or locker to work with the shafts you desire to use. 28 and 31 spline are very common, as the 28s came in 66-77 Broncos and the 31s came in F trucks and 78+ Broncos. I.e., you can't use the same Detroit with 28, 31, 35... I've...