Low speed stumble, rumble issue

I have a 2009 Access cab
4 cylinder
5 speed stick
Rear wheel drive
About 80k miles and I've owned since new.
About a week ago. I noticed that when rolling to a stop with the truck in neutral. Clutch in or out. The truck rumbles a bit at very low speed like it would if I was in like 3rd gear and had not yet depressed the clutch.
I mean that the best way I can describe it. No usual noises at all.
If I drive in gear at low speed I feel it. If I put the tranny into neutral it's the same. While rolling slowly. And I mean 10 miles an hour or less.
I jacked up the truck. No bearing play or noise. No funky tire wear or damage seen. Wheels spin freely. Rotors are about 20k miles old and still seem perfect.
U joints seem tight. No play. No noise.
I opened the hood. Placed the parking brake on and let the clutch partially out a few times to look for engine or transmission mount failure. Non found.
Truck operates normally at every other speed. Just feels like that when driving slowly. Brakes applied or not. And brakes themselves work great. With no pulsation felt through the pedal.
Ok, collective.....
What are your thoughts?
Because I've run out of them.
Thank you!!

 

Wheel bearings would be high on my list assuming rumble is from the chassis and not engine. Don’t rule out the rear axle bearings

 

Remember how I mentioned that it felt like a stumble?
Like I was stopping in 3rd gear without pushing in the clutch?
It seems to be a "bad gas" issue that I bought at a no name gas station. I noticed that if I have the A/C off. The truck didn't do it at all...
So I dumped in about 16 ounces of my trusty K100 fuel treatment and in less than 3 miles, it's just totally gone.
What a strange event.
It really had me scratching my head.
I DO RECALL some engine pinging while I was testing the truck out. I should've put it all together then.
Thank you.