You can replace it with a new Shimano 9 speed MTB derailleur. Shimano changed the cable pull when they went designed the 10 speed MTB derailleurs (labeled as Dyna-Sys) so only a 9 speed (or earlier) will work with the shifters you are probably using (I assume indexed) . You can use either the

Shimano RD-M771 XT 9 speed Rear Derailleur - which has what I think of as a classic look

OR

Shimano RD-M772 XT 9 Speed Rear Derailleur - which has the "Shadow" lower profile body with direct cable routing

They are about $60 to $70 new. Both have the traditional "top normal" return spring (i.e., the derailleur moves to a smaller cog when you loosen the cable tension). Make sure you get the correct length (GS medium cage or SGS long cage). If in doubt go with SGS.