Thing is any of these things can cause the upshift. If the engines working too hard the throttle position causes the shift. This would also cause the mileage so a clogged cat could be the cause..

 

Thank you all for the suggestions! I guess I'll start with the cat. and see if that sorts it out. After that, I pray it isn't a transmission problem. Any additional comments would be most welcome! This is my first Jeep, and once I have it running good I will be very happy! Michigan winters were made for Jeeps! (or the other way around.)

 

Highway is when the engine is really working. It's like trying to push a huge brick down the road at 70. They aren't very aerodynamic. Another big and overlooked possibility is your brakes. Make sure they aren't dragging. Your tps. Is another possibility, wouldn't go there yet but its a thought.

 

replaced cat. convert. and....what do ya know!! problem solved! Amazing how much difference it made to performance and hopefully fuel economy. Still on the first tank but am expecting to get at least 3-4 mpg. better.