Rear derailleur on this set I have one KHS sixfifty 500, I want to upgrade my bike so I have already bought 

Shimano Deore XT M771 front derailleur 
Shimano Deore M590 shifter 

So I want to know your suggestion to rear derailleur and cassette? 

I want to know if exist rear derailleur with lock. 

I was planing to buy Shimano Deore XT M772 rear derailleur but its hard to find in Brazil, so has a model like this? 

Thanks
Indeed as per the comments the 'lock' mechanism on Shimano rear derailleurs (to reduce chain slap) is called "Shadow Plus". Whilst this was initially a feature of the higher end models (XT / XTR) a few years ago, it has since filtered down to some lower models, such as SLX and Zee (although I think not yet on Deore ... anyone correct me?!)