Yeah, it looks like the ball and pin need to be further inside the diff cup, so the only way I can think of is to find a longer drive cup, such as a savage transmission drive cup (6x13x20) HPI 86276 compared to the extended revo ones (6x13x19) or to shim the diff as close to that side as possible, as you are trying to do, but this may lead to problem occurring on the other side. Does the ball and pin of the cvd just fit in the cup on that side as well as the side that broke? Is the photo of the other side that hasn’t broken yet?
You could always try regular diff cups and not bother with the stub axles and the extended revo drive cups. Or you could even use a wider diff of the same gear ratio as the one up front, there is no problem with that one so no need to buy two, to move the drive cups out a bit more.
If you are using the FLM Ultimate Rear Hybrid Bulk have you tried using the diff cups that are already supplied with it, installed in the diff in the photo on:
http://www.rc-monster.com/proddetail...LM17850&cat=31
It is not ideal but you could enlarge the pin screw hole in the diff slightly for the stock sliders. They are pretty good from experience but I have broken two due to it rubbing on the shock spring retainer and wearing thinner.