compatibilty of 10 speed dura ace bar end shifters and xt 10speed rear derailleur
Your bar-end 10s-shifters must have the same index (cable-pull ratio per shift) as the normal STI 10s-levers: 2.3mm. That leads to the incompatibility with the MTB 10s-derailleur, which is build for the 3.4mm cable pull of Shimanos 10s MTB shifters. Using a 9s-derailleur, which requires 2.5mm cable-pull, brings you close to the index of your 10s-shifters, so that kinda works. Adjust it to align perfectly with the middle gear and the outer ones wont't be too far off. (ref: artscyclery)

If you want to use your 10s-MTB-derailleur (probably for the shadow+ capability?) with your 10s-RR-shifter, you'll need to change the derailleurs cable-pull ratio. You can do that either by re-routing the cable through a small hole in the derailleurs lever (ref: mtb-news.de and in english pinkbike). Or by using a special excenter-pulley (ref: Jtek ShiftMate) which translates the cable pull-ratio to the needed 2.3mm.