Tom, I have been using a solidly mounted rear end in mine for many years (racing years) with no cracks. My only concern, and I've never brought this up before, is if you use solid mounts on one end (either end) of the differential, then you should use them on both ends. The reason I FEEL this should be the case, is that the solidly mounted end is not really designed to flex, but if the other end is moving around, then obviously the solid mount and/or the aluminum case has got to flex too, and this, to me, is the setting for something to crack or break. Even if the movement is a fraction of an inch, in time fatique will take it's toll. Again, this is only my opinion, but you should go all the way with either one (solid mounts) or the other (flexable mounts), but not both. IMHO mixing is asking for trouble.