Shields can opt out after 2016 (and the SP market in free agency sucks this winter so there's a chance he will). Even if he doesn't opt out, he's a much shorter contract and likely far more tradable. Plus, Preller wasn't as derpy this past winter as he was two winters ago.
I honestly can't think of one team who'd trade assets and take back Tulo at 4/80 for his age 32-35 seasons. We'll have to find the equivalent of Reyes and take that contract back, most likely. A part II of a Vernon Wells trade would be great, though.