I have a 2011 4.8l with 4l60e And for the past few months now the truck has all of a sudden started to have a hard 1-2 and then a bit so bad 2-3 shifts it feels as if it is jerking into gear. This usually starts after about 30min to a hr of driving and will continue to do until I shut the truck off for a few mins and restart it then it good again for a bit. Sometime it will do it every day but then there are times where it won’t do it for weeks not sure what it could be.
transmission fluid and filter where both just recently changed, and there is no check engine light on

 

My 09 has done this since I bought it at 70,000km 3yrs old.
It never got worse so I just left it. Transmission has been flushed, fluid is good and full.

Don't quote me on it, but I think this may be partially to blame on the slack in the drive train that these trucks typically develop, the same slack that causes the random bump or thunk when you come to a stop as the drive train takes up slack.

 

same deal on my truck man. i dont know if its the trans or what

 

#08-07-30-021E: Loss of High Speed GMLAN
Communications, Intermittent No Crank, IP Gage
Fluctuation, Intermittent Door Lock Cycling,
Intermittent Chime Operation, Various IP Warning
Lamps Illuminated - (Jun 7, 2010
This service bulletin listed here list some of those issues with pictures also.