Hi Ben,
JCconcepts only sell the JC specific parts as spares, AE parts like the diff plates ect should be sourced from your LHS
B44 diff's can be used but the inners are slightly different from the NTC3 diffs that come with the BJ4. Main/thrust balls, crown gear and thrust washers are the same - outdrives and diff plates are different on the B44 diff. Note sure about the screw/spring/t-nut as I've not had a B44 diff apart.