Make what you want to know clear.
As to shortening the diff - unless you have a jig to keep it all aligned and you REALLY know how to weld I think you are asking for trouble doing it yourself. The thought of the whole axel assembly passing you on the freeway is scary...........
I'm guessing that if you shorten the diff, you need shorter axels as I don't know how much spline you have to play with. If they have to be shortened, recutting the splines (assuming there is enough meat) will weaken the axel.
In short, get a proffessional to do it, find a shorter diff, or don't drive it so hard :wink:
Happy now