I am wondering if I can use a SRAM XO 10 Speed Rear Derailleur with a pair of 9 Speed Grip shifters / 9 speed chain and cassete? I believe the pull ratio is the same, so if I set the limit screws correctly I should be able to use it as a 9 speed derailleur, correct?
Thanks.

I was able to do it with a 7 speed shifter & an 8 speed cassette, so I don't see why it wouldn't work.

I would say so, as well; the indexing is done through the shifter, I don't see SRAM doing any sort of 'special engineering' to keep the 10-speed derailleurs separate from the 9.

To the OP, it should work, provided the pull ratio is the same 1:1 that SRAM loves so much.

Sam, your set-up worked because when going from 7spd to 8spd, manufacturers added more space with a longer freehub. When going from 8spd to 9spd to 10spd, Shimano/Sram just crammed more cogs into the same space by making the gaps between the cogs smaller.

Yes, a 10spd derailleur will work with 9spd shifters. A derailleur is a "dumb" component and basically a spring as long as you set the limit screws.

Not in Shimano's case. 9spd mountain and 10spd road are interchangeable but 10spd Shimano mountain uses a variable pull ratio and won't work with your 9spd shifters.