FWIW I've been having problems with my XT 9 speed as well.
My Jamis Aurora Elite (2010) came with a triple 9 speed setup, Tiagra (4503) up front (50-39-30) and XT in the rear (11-34) with a M771 RD. I bought it used and it needed a new chain and cassette, which I promptly replaced. However, I was still having issues smoothly shifting through the entire range of gears and paid a local shop to align my derailleur hanger, thinking that was the issue. Still no joy. Also had a lot of chain slop in the smaller cassette rings.
I found a like new takeoff of the "upgraded" M772 shadow RD. Installing this alleviated some of the chain slop I was getting in the smaller cassette gears but exacerbated my issues shifting throughout the entire range of the cassette. If i indexed the drive train to behave well on the large cassette rings, it would rub on the small ones and vice versa. Took it to a local shop and they thought my derailleur cage was slightly bent, and they 'fixed' it. Still didn't quite ride right.
Well, I figured, if it was slightly bent it would never be perfect. So I bought a brand new Deore (not XT) M592 RD, which is essentially the same as the M772 but more or less commonly available. Still having the same problems - I can index and shift well great through 2/3 of the cassette but then the last couple gears either start slipping or fail to shift at all. Turning the barrel adjusters just moves the problem to the other end of the cassette. A funny thing happens in the 8th cassette cog where the chain sits up on the edges of the chain instead of slotting into the grooves.
Nothing looks bent, everything adjusted per Park Tool video recommendations, all shift cables and housings and cable guides replaced with new components, shift controls oiled. Can't figure it out other than thinking something on the frame may still be tweaked.
I just ride my bike like it's a 6 speed =/