Originally Posted by phunk

All of this. ^^

A differential brace is a bit of a hack created as the lazy mans way to mitigate differential movement. Will it massively reduce movement in the rear differential? You bet. But like mentioned in the quote above, you are now focusing nearly all the stresses to a single mount at one corner of the differential. If you truly wanted a solid mounted differential, the proper way to do it would be to replace all 3 of the original mounts with solid bushings. This would keep the stresses spread out rather than focused at one bracket at one corner of the differential.