Engine revs 200 rpm higher on highway- shifting to lower gear?

Engine revs 200rpm higher on highway

2000 2DR 4WD 2.7L auto trans 120k miles, owned 15 years

I recently moved and it now requires an 80 mile commute, mostly on the interstate. Before, I pretty much only drove my Taco for short trips, so I don't know if this is a new issue or not.

>55mph and I go to change lanes I always add just a tiny bit of gas. I never even knew I did it before I noticed that the engine RPMs go up by 200. Usually from 2200 to 2400. I know it's not me solely responsible for that 200 RPMs because it doesn't do it when I'm <55mph and changing lanes. The extra RPM seems disproportionate to the throttle increase I am doing.

I assumed it was changing to a lower gear when it was doing this but I don't think it is. When I turn off overdrive, which I assume takes it from 4th gear to 3rd gear, the engine goes well over 3000 RPMs. So it would appear to me that it's just the engine revving.

It does not do the revving below 50 mph, or when I'm going up a hill and give it more gas to maintain the same speed. Only on level or downhill grades.

I am getting nearly 21 mpg consistently, so it's not really affecting performance, it just seems like it could be a symptom of something more serious.
I admit that since I've only typically driven it 3k miles a year (hence the low mileage) I haven't done much engine maintenance other than oil and air filter.

 

That's just the torque converter. It's kind of like a 1/2 gear where it lets the TC slip a little. If you continue to put your foot in it, it will grab the next gear.

 

Might be the lockup clutch disengaging, but I'm not sure why it would be unlocking under those circumstances.