2015 Silverado 4.3 issue

[Jw1424616]

I have a 2015 Silverado with a 4.3 that has a problem I cannot figure out.  The truck is getting awful gas mileage recently, 8-13 mpg highway.  Was getting 22-27 mpg, it also seems like a jerking feeling in the seat like I have a trailer.  For awhile it’s been hard to make it shift into the 6th gear, I have to keep letting off the gas.  No check engine light.  139,000 miles, new tires, new brakes and rotors, full tune up, cleaned mass airflow sensor, ran injector cleaner through it, transmission runs about 187 degrees, fuel pressure 55-60.  This is a base model single cab 

 

thanks

[Grumpy Bear]

A guess? Transmission. Ever change fluid in it? 

[silveradosid]

try cleaning the throttle body