I have '09 YZ 48mm USD forks on my XTR, xt and yz steering stem are 30mm in the lower part, so you can use xt OEM lower bearing.
On top the yz stem is 28mm and the xt one is 25mm. I machined the yz stem to 25mm to fit the xt OEM top bearing.
I had to do new tread to fit the xt nut (the one that presses the top bearing) and the nut that goes in top of the superior clamp I used the yz one.

You can fit the xt stem into the yz clamps too.

Sorry for my english but I'm in my third beer in a bbq and now I wonder how I can remember all this measurements!
So, if finally you go this way ask me whatever you want.