I'm glad this thread is getting additional comments on it because I have been reading through it lately. I just bought a 15 GMC Sierra with the 5.3 6l80 with 22,000. I love everything about the truck but the transmission. Similar issues to what you guys are having, but I don't think it is mechanical, at least with mine. It's like it can never decide to lock/unlock/shift down. And inevitably, it chooses the highest gear with the lowest rpms, which for a guy coming from a 04 6.0 with a 5 speed standard is infuriating. I can't believe how awful these transmissions are, especially for how low of miles these trucks have. I think it all goes back to the TCM and adaptive shift patterns. I'm hoping one of the gurus from this site will give us some answers, I'm ready to pull the trigger on a trans tune before my first payment.
