When first installing (or overhauling) Shimano rear hubs, I usually pull the freehub, lift the seal off the back side (where the splines are that engage the hub body) and then heat the freehub with a heat gun. When the old lube flows out, I drip in enough Phil's Tenacious oil to keep the things utterly silent. I love silent coasting. I have had no trouble doing this with Deore LX freehubs and others, but the FH-M8000 XTs I've got don't seem to want to give up the seal. Usually, gently picking the rubber seal with a large (not too sharp) sewing needle lets you pop it right off, but these don't seem to want to budge. Anyone out there pull one of these rear seals and know how it's done?
I would take the hub to your local bike repair shop. More than likely they will pull out a special tool just for this issue of your seal not wanting to unfasten itself.