I have the same problem on a 2015 GMC Sierra 1500, 5.3, not sure the model of the 6 speed transmission. 127k miles and it felt like it was missing. I was 400 miles from home and made it home Friday, this morning taking it to my mechanic barely made it 5 miles. He said solenoid was $1200 and probably the torque converter was toast. Going to cost me $3000 +/- to rebuild. I almost traded for 2020 2 months ago, but I love the 2015, its been a keeper until now.