I have not pulled the case cap off the ARB yet, but I can see the broken pin. In this carrier it appears there is a pin insert under the non-rg side cap and retained by that cap.
It's been a while, but I think you're right. It's a solid roll pin and there should be three? of them... One in each small one and then one in the large one. It goes all the way through the pin and into the center of the case. I wonder if that pin busted in half or fell out? My guess is broke in half? Because how could it fall out if the end (cylindar) cap is still in place?
I've replaced main parts like that and it wasn't all that expensive - cheaper than buying a new one anyway... I want to say the two parts that hold the ring gear - flange cap and diff case were something like $180? The flange cap was in stock up here in Seattle, but the diff case had to be made and came from Australia - 10 days'ish?
As for the differences between 10.25 and 10.5, I'm pretty sure all of the differences in the diff are in the pinion - I think it's the inner pinion bearing is the only thing different. So if you put 10.25 gears in a 10.5 housing, you use the 10.25 master install kit with the 10.5 inner pinion bearing. But point being I know the ARB is the same for both.
As for short or long pinion, you can run the long pinion gears (in a short housing) and just replace the yoke. I believe 93 was the first year for the long yoke.