When building my new FT 4X4 SC10 I discovered both deep diff gears were the wrong parts. The deep diff gears were in the bag with the rest of the parts. The problem was the seat for the large O ring on the inside of the diff case was too small. I did not notice the issue when building the diffs up the first time, but once installed there was a lot of binding and they did not feel right. I pulled them back out and opened them up. That is when I saw the very outside edge of the O ring sticking outside of the washer. In the pics I was able to turn the outdrive to give a worst case scenario for the pic. I also show the wrong diff on the left and the correct one on the right. I must say AE support is great. I called them and they sent out the new diff cases right away. I got them 2 days after I placed the call. Once I had the new parts, I took the pic showing the correct and incorrect parts before I sent them back to AE. I'm posting this in case there are other wrong diff cases in some kits.
Bah, since I'm new, I don't have enough posts to show pics.
Anyway, if you are having diff issues, this is something to look at.