Can someone tell me if the third members from Ford 9" differentials are interchangeable between the many different years of housings? Or does the third member have to be the same year/vintage as the housing?

 

The only differences you should run into is the different axles (28 vs. 31 spline) and the pinion yokes are not all the same. Neither of these things will keep the third member from fitting in the housing though.
Also some of the earlier rears had the fill plug in the housing and no plug in the third member. Still would fit but could end up with no way to fill/check.

 

Some good info;
https://www.ford-trucks.com/forums/2...xle-swaps.html

More info then you ever wanted to know;

Kevinstang's Ford Nine Inch Differential Page +

 

Thanks guys for the info I appreciate it. So if I understand correctly any third member from a 9" 57-85(?) will work in any housing with the only difference being the axle splines either 28 or 31. Okay I think I got it now!