Lately my truck can't pull any hill in 6th gear and most of the time not even in 5th. It will drop about 5-6 mph then drop gears. A few times it has dropped 9 mph and then goes back and forth from 5th to 4th just sputtering. I have changed the fuel filter and cleaned the air filter with no change. I haven't done anything different to the truck since this started happening. It never had these issues before. Any Ideas?

First guess is winter fuel? Not sure how long they keep it at the fuel stops up there.

Those wheels and tires are probably a 1 if not 1 1/2 MPG penalty at least so I would expect you shift rather quickly into 5th. You did the easy stuff now time to go over the balance rates and other stuff. Think winter fuel goes until April. I know my mileage never completely returns until sometime in May. May June, July August and September is my best mileage. Once October hits I loose a little because the truck likes it hot for mileage. Being in Wyoming I would loose the Ram air as from my experience diesels like hotter air for best mileage.

Try pulling the banks off and see if it does it still... I'm betting its defueling ya.

When I get back home in 2 weeks I'll try it, but it didn't do that to me until recently. Around the 1st of the year.