Which rear mech to replace sram X-9 10 speed 2x10 I have a Whyte 905 2011 with a 12-36 Sram cassette and an X-9 medium cage rear derailleur. It's not been attenuating well for some time despite efforts to adjust. The bike shop couldn't fix it. So i'm thinking i need to replace. The X9 appears to be discontinued (and forums suggest people had problems with them) but i could get an ebay replacement. Sram  now appear to be going with the GX-Type 2.1. Is that the mech i should use? Thanks!
You need to purchase a SRAM 10 speed rear derailleur to ensure compatibility with your shifters. SRAM previously made a wide range of derailleurs for X5, X7, X9, X0 and XX but have limited their range to GX and X0 following the market shift to 11 speed drive trains. 

GX has replaced the old X7 and X9 price points.

Your options are limited as SRAM and Shimano rear derailleurs/shifters are not compatible.