mega 9 ultegra or XT
Will an Ultegra long cage rear der work with an 11-34? or is it the XT, XTR line of deraillers only? If so, why?
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.