Dropping the diff is actually not as hard as it sounds. The hardest part is just finding the drop brackets. The Calmini ones are not available separately. I think there are maybe some available from other vendors, maybe boondox, I'm not sure. But once you find the brackets, it's a pretty simple matter to change them in inplace of the stock ones. Basically support the diff with a couple jackstands, unbolt the brackets one at a time and replace them with the drop ones. That's it. You wont need any shaft spacers or custom work or anything.
I can tell you from my own experience with my Calmini pieces that the 2" suspension lift kit includes drop brackets that lower the front diff 1" and the 3" suspension lift kit lowers the front diff 2". That's another reason that I say don't go higher than 2" of suspension lift without lowering the diff to compensate.
Stock, the lowest part of the front diff rides almost an inch above the lowest edge of the crossmember it's bolted to. With the Calmini 2" suspension lift kit, the lowest part of the front diff rides about even with the lowest edge of the crossmember. With the Calmini 3" suspension lift kit, the lowest part of the front diff rides about 1" below the lowest edge of the crossmember. Since I have my skidplate bolted directly to the crossmember and extending forward all the way to the bumper, I didn't want to have to cut a hole in my skid for the diff to stick through and possibly take damage, so I stuck with just a 2" lift and 1" drop brackets.
