I welded the rear diff on my 1984 Toyota pickup. It worked just fine, but it was an off-road unit only. Funny thing was that my stock, open front diff blew and I got rid of the rear and it never had an issue.
When I upgraded to a larger 3rd member for my Toyota, I welded the rear diff again. This time it didn't turn out as well, as there was some cracking after welding. I don't think I had it cleaned well enough.
I still used it, it worked and I haven't had any problems yet. Mind you it's low speed, low power, light weight and I don't drive it very often. I'm also just planning to put a spool in if my welded diff goes kaboom anyways.
In short, if you want to have a bit of fun and are planning on an upgrade anyways, then I would weld it up. But if it's a daily driver and you can't afford to have it sitting because a welded rear diff exploded, then I wouldn't bother.