A lightly used long cage derailleur would be cheaper than parts, if parts were available, which they aren't.
All new (2012/2013) XTR derailleurs are the new MTB 10 speed type, which will not work with your 9 speed shifters and cassette.
What are you max/min cogs and chainrings? Shimano publishes the 'capacity' ( (max cog - min cog) + (max chainring - min chainring) ) that their derailleurs can handle, and if your setup is less than the published capacity, it will work... if your setup is more, however, it either won't work or you will have to select chain length to allow shifting into the bag/big combo and avoid small/small.