I have a 19 1500 LD LT with the 6l80. My problem started around 4000 miles and has been in the dealer three times for this problem. I have read a few threads related directly to this topic and haven't seen a resolution posted.
The main problem is the delayed shift (3-5 seconds delay) into drive from park or reverse on the first startup of the day and when it does engage it will bang. Now, if I forget to allow the 3-5 seconds to lapse and immediately press the accelerator the transmission also shudders until it fully engages. It does not do this when I shift to reverse if reverse is the first shift of the day.
I do not notice any other shift problems, just the initial shift to drive of the day.
Darrel