2004 Chevy Tahoe, 5.3 LS V8 with an auto 4L60E. Original trans went out so we got a used one that was pulled right from another Tahoe with the TC. 30 day warranty guaranteed, mechanic said he pulled it that morning after driving the vehicle himself. Both the original and the used are stock trans. Now the issue; the used trans hesitates after reaching 40mph, at 6k rpm. It struggles a little bit but once it gets past 50-60mph, it flutters/hums as if you’re taking your foot on and off the throttle, but we are not. Pedal down, fluttering happens. We swapped smaller tires and it’s less harsh on the transmission but it still hesitates. Bad trans, torque converter, solenoid??

 

I'm sorry but your description of what the trans is doing is very vague and not very helpful. Does the transmission shift gears? What gear is the trans in when it hesitates? What do you mean by hesitates, trans not shifting? To get past 50-60mph does it shift after the hesitation? Any scan data? Everyone needs more useful info to be able to even start guessing what may be wrong.