9 speed shifting problem
 

I have two 3x9 speed bikes, both mountain. One shifts perfect, one doesn't like the 4th from the bottom cog and shifts past it every time on the stroke up or down. swapped wheels and the problem shifted bikes swapped cassettes and the problem stayed with the XT CS M750 9 speed cassette. (used when it came to me) what are the odds that someone respaced it for other than mountain use?

The cassette that works properly is a Sram PG-950, cheap enough, I guess that's an option.

If the cassette is used it is probably worn out at least in that cog, I would just toss it and get something new. There is no more troubleshooting needed it is an old used cassette of unknown usage. XT or not it is a wear item that just needs replacement. Luckily you can still get CS-M770s which is the latest XT 9 speed cassette if you want to keep it XT otherwise PG-970 or 950 will work.

Check your hanger alignment and cables.

It could be that someone respaced it for something other than mountain use. I agree with Veganbikes, get a new XT CS 770 with the same gears than on the one that shifts perfectly. There is plenty of XT stuff for sale on ebay, you will certainly find what you need.