91s are 31spline.so does that mean that the 91 carrier is also a 33 spline? So do you have the arb part number by chance?
I'm pretty sure the part number for the 33spl H233B's is RD24 (confirm this before ordering) http://www.arbusa.com/alac/alsp/0-00.pdf . Whats intresting to note is that if the part numbers are chronological, the 31spl locker would be the 4th one they made (before any dana stuff). I'm pretty sure they are since the rockwell locker is RD145 and it was just recently released after a long development cycle.
PS I've had nothing but bad experience from 4x4parts/AC, if its an ARB any normal dealer should be able to get it.