You will need a 9-speed rear mountain derailleur to work with your STI shifters. Shimano 10-speed mountain type rear derailleurs have a new actuation ratio to work with their new 10-speed mountain shifter cable pull. Be sure to get a "high normal" type RD so that the shifters work the same way as with the road derailleur. A "low normal" RD would make the shifter work backwards which is OK but most road riders don't like the change.