Transmission Problems at high speeds

[Guywhoworriesabouthistruck]

I’m new to the forum but have researched this issue without much luck. I own a 2014 Silverado 1500 5.3l v8 with around 124,000 miles on it (I drive around 30,000 a year). Just this last time I was driving from Kansas to Texas I noticed that the truck was reluctant to go to 6th gear going 80mph on the highway. To get it to go into 6th I had to slow down first then it was go into gear and I would accelerate back up to 80 in that gear. However every time it switches back to 5th it had this problem. Occasionally (twice)  on the drive it was switch between gears back and forth and create a jerk and then once it was in 4th I believe and the RPMs shot up without my speed increasing very much. What seems strange about this is that my transmission seems to work perfectly in town driving and even up to 65 mph but at 70 and up I got these issues. There was VERY strong head wind that day  and I was wondering if my transmission is slipping, something else is wrong, or the head wind was really affecting everything?? 

 

Also so I did just get new tires the day before switching from 275/70r18 duratracs to 275/65/r18 4ply trailrunners (55 pounds per tire to 40) not sure if that would change things or not 

 

thanks