My 16 4x4 3.5 eco has just drove us from AZ to New England while towing 4k or less and a decent amount of payload.

Now that I'm unloaded I notice a lugging in 5 and 6th gear- or 55-70+ mph. Its really pedal dependant. As in it will easily increase or decrease the hum/vibration. If I take my foot off it goes away. If I shift into neutral it goes away too. Under load or a hill on the freeway while the truck has not downshifted (yet) it is probably the most noticeable.

So far I made sure there was no obvious play in my U joints. But other than that I'm convinced my 2700 mile drive triggered something...any ideas. She runs well otherwise..

 

Notice you see it go away in Neutral, light throttle application.
That enables the intake vacuum to be at it's highest level.
This affects the front drive system by keeping the front lock up >open in 2wd.
In lower vacuum levels higher throttle levels the front hubs may be trying to engage is the hum noise you hear.
I would be looking at the hoses for breaks, cracks and the check valves for leaking.
If none found, one or both front hubs may need to be to replaced.
Good luck.

 

Silly question... you don't have it in 4A or 4H, do you? Sometimes it's the little things.

 

I'm definitely in 2wd but that is a great suggestion to look at. I once knew that system very well.

Because I just moved i probably don't have my vacuum gauge available yet. I will test my 4wd..although if only one side was leaking- then I engage 4x4- now i have 2 weeks engaging and id hear a difference anyways.
the noise and vibration is so settle. A passenger would never feel it...although my wife noticed it once I pointed it out-- so she says...lol