Trans shifting later/lower power/bad linkage?
 

Hi everyone. It seems that this truck has been having issues lately with power. Just feels like I am dragging something and the trans is shifting a few hundred RPMs later than usual or more even with the same throttle settings. The only real issue I had lately was with a bad starter, but it can't be related. The only 2 real variables I can find between now and two weeks before is a few hundred pounds of cargo (not causing it. I emptied the truck and that isn't it) and my cargo box (both empty and with 100-125 pounds of cargo). Now call me stupid, but can a cargo box cause enough aerodynamic drag to not only make it feel bogged down, but make the trans shift late? The fluid is fine and when I pull over to check the trans temp at the cooler, it is between 100 and 140 under normal conditions. No check engine light. I am attaching a photo of the truck as it is now. Box and truck are both empty. I noticed that when towing a trailer or with a carrier on the roof, it changes the trans fluid change intervals to 15k (like I am already doing), but how does that change anything else with shift points? Gas mileage only went down between 1 and 3 mpg with the box on the truck.

I don't know if this is related, but something is wrong with the shift lever, specifically the D and 3 positions. When I was in the mountains on Saturday, I downshifted into 3rd to get up some steep hills to lock out overdrive to keep the trans cooler. Once the road leveled out, I "clicked" it into D again, but it seemed that I was dragging a boat anchor like it was still in 3rd. I pulled over, shifted into N and then D and I heard TWO clicks as if it was still in 3rd. Then I continued and it was normal and went into overdrive and locked the converter. Weird. Could I have a bad shift linkage? Can it be adjusted?