11' Denali Duramax problem
 

So I had the Shift inhibited message one day and the truck wouldn't go into any gear. After sitting awhile it started working fine again. Next day I added some tranny fluid because it was low, took the truck around the block and it was shifting ungodly hard up and down. The next day it was back to normal again so I drove it around the block a few times to get the tranny up to temp. Tranny fluid checked out good so I turned the truck off. I pulled the plug on the transfer case and fluid level in it seemed good. Started the truck up about 30 minutes later and it wouldn't go into gear again, was showing the shift inhibited message again. When I shift out of park you can hear the tranny/driveshaft click like its going into gear but it won't move. Does the same thing in 4hi and 4lo. I unplugged the TCM and plugged it back in and still nothing. Also while truck is in park it seems like its idling high to me and about once every 20-30 seconds it kicks down for a split second as if something is trying to kick in, almost like turning the AC on. Has anyone that has had to replace the TCM had these symptoms? Or can someone point me in the right direction? Also truck is throwing no codes. Any help is greatly appreciated