I think that it was just a full 4.37 3rd diff from an 81,
'81s were only available with 3.90 gears. And I wouldn't use the diff. Why chance it? You're going to be in there with the gears anyway, why not do it right while you're there. Trail failure sucks for everyone because they either have to tow you out, or spend hours helping you fix it on the trail. If you're not going to fix it right yourself, fix it right for the other guys.
