Problems with towing 2001 z71

[ChevyJakeZ71]

when I first got my truck it pulled my 2004 zx225 skeeter just fine. Not great but ok now it pulls like [censored] feels like im pulling something that weighs twice as much when I press the pedal all the way to the floor it won't rev the engine over 2700 rpm thus almost zero acceleration this happens between 3rd and 4rth gear 40-60mph and forget towing it up a hill. It has the 5.3 323 vortec and the 3:43 gears. IV had it looked at and they say everything works and runs great any body gave any problems like this and what did yall do to fix em? Transmission shifts fine and everything has been serviced. Thanks Jake

[grayblazer]

Do you notice any change in performance when driving the truck without the trailer? If not I would start looking at the trailer such as dragging brakes or bad wheel bearings.

Is the transmission downshifting from 4th to 3rd?