Change your tranny filter regularly!
After having the tranny rebuilt at 115k as many others have done, we started to have problems with the OD fully engaging. The final output drive didn't match up with the input shaft.
I changed out the tranny fluid, using a pump system through the ATF filler hole. That seemed to help, clearing the warning light and shifting properly again.
Within weeks, here we go again...I tried to remove the tranny pan but with the subframe in the way, it was almost impossible to access that one last bolt!
Another week and my wife says it's starting make noise. Oh great!
Now I test drive, come back for a minute and when now shifting, it won't move in any gear.
I let it sit for an hour and now it moves again?
I know when the filter gets clogged, it won't supply the hydraulics of the tranny to engage or release the clutches, etc.
I have it towed to my local shop, pull the filter and change out the ATF, and now back to all good. Knock on wood.
For all of those with this GM tranny, knowing the issues, get your ATF and FILTER changed regularly. I'm doing again at another 35k.