Tow Mode raises shift points to prevent it 'hunting' as much for the correct gear, and it has a more aggressive grade braking function. You can switch it in/out of tow/haul anytime to see if it makes a difference, without worrying about damaging anything.

I had (have ) same issue with my ‘03 Silverado. I checked tranny fluid and found it to be over filled. I siphoned out about a pint. That helped a lot but didn’t totally alleviate the issue. Since then, I lost 4th gear and starting from a dead stop very sluggish. CIL illuminated and threw codes P0756,753,and 758. Replaced 2 shift control solenoids and ran great for 7 days. Yesterday I felt a slip shifting into 2 nd gear and lost 4 gear again. Now, starting at a dead stop even more sluggish than before still no 4 th gear. Tried manually shifting from 1st to 2nd to 3rd, etc. found no 2nd gear even when shifting manually. Feels like trying to start off in 3rd gear when in drive but only minimaly faster if starting in 1st