There is a TSB for 08KK,thats sounds close to what your saying.See if this it what you think its doing,and if it is see if the dealer can do the TSB for you,:
SYMPTOM/CONDITION:lockup speed to 50 mph.
Some customers may complain of a drone sound or a condition that feels like an irregular,
very short, slight deceleration as if the vehicle is being "tugged" from behind. This
condition exists between 1000 and 1600 rpm, 38 to 45 mph at constant speed. This
condition will be more noticeable during a light acceleration within the 38 to 45 mph vehicle
This new software update has removed 3rd gear partial lockup and raised the 4th gear full