No, the Ultegra or other Shimano road derailleurs won't work. You need the XT or another mountain derailleur.
Shimano says their road derailleurs will work with up to a 27 tooth cog, although Sheldon Brown and others say they can go up to 30. I believe the side of the pulley cage will hit larger cogs. Pulleys on mountain derailleurs are spaced farther from the axle to accommodate larger cogs.