Probably. I've done XT M771 9 speed rear derailleur installs on Shimano 10-speed groups several times. It works fine, but the shifting is not as quick as Ultegra. Don't try the shadow derailleurs--we could never get them to shift well.

I just did a swap out on my wife's Ultegra bike by replacing the Ultegra RD with a 105 RD-5701-GS, which has a rated max cassette size of 32 teeth, and a 37 tooth chain wrap capacity. No difference in the shifting.