Just had my transmission rebuilt and hooked up an empty 26’ enclosed trailer. Immediately got hot (280 degrees), wouldn’t downshift and then would just rev in drive until it cooled down without any movement. The next morning tried without a trailer and when the truck normally would drop into overdrive, it downshifts from 3rd to 2nd gear. At 45mph, light throttle or the slightest of hills will cause the truck to downshift to 2nd gear. It will no go faster than about 45mph. Doesnt matter if the o/d light is on or off. Fluid is full

I feel some shuttering when ascending any elevation

Is this the torque converter?

 

Fluid might be overfilled which leads to aeration.. but the rest sounds like a rebuild gone wrong to me.

 

I'd go back to the tranny shop with a few questions/concerns before you mess with it.....