I'm wondering the same thing, diff back could mean 1 of 2 things:
1- "turbo back" off the front diff, Which is from turbo all the way to the muffler, there's no turbo in the euro so that's out.
2- "axle back" from rear axle backwards, so effectively just 2 mufflers and the connecting pipe, so HALF a catback so to speak (because its got no mid pipe). Gosh I'm hoping he's not charging a whole lot for what's basically 2 mufflers.

