if you have a limited slip differential then I would venture to guess that it is the LS cluth packs shuddering. Mine did it too, the harder I launched (or starting off with heavy load) the more pronounced it was. My dealer also told me it was not bad enough, and then told me it was vibration from my Bassani exhaust.
I went to another dealer and they replaced the LS clutches and it has been smooth-as-silk ever since.