I had the exact same problem and after disassembling the integrated shifter I realized that I was indeed in over my head with the complexity of the assembly. I spoke to the mechanics at my bike shop and they recommended replacing the entire assembly. They noted that the probability of getting it to work reliably after the repair was low and I didn't want to deal with an on trail failure. I found this article
XT Shifter Repair which will give you an idea of what you're up against. I managed to find some new old stock Shimano XT 8 speed shifter set on the Internet for around $75 so I now have a backup should my rear shifter fail. Good luck and if you find you need some internal part let me know, I'm sure I saved the old assembly.
Be Self Propelled