Shimano 10 speed road and 10 speed mountain are incompatible.

But, you can totally use a 9 speed mountain derailer with your 10s Shimano road shifters. It will index perfectly.

My favorite rear derailer for this situation is the Deore XT RD-M772. It can fit a 36t cog, and the Shadow design is nice and low-profile.

Remember that if you put on a larger cog, you'll have to replace your chain with a longer one.