4Runner Shudder on slight hill, 20-30mph?

Hey all, could use some help.

My dad has an '07 4Runner, 2 wheel drive, SR5 with about 150,000 miles. Within the last two weeks, it has developed a slight shudder on small hills around the neighborhood that start anywhere from 20-30 miles an hour. When I drove it, it happened on about small (like 20 yard long) uphill stretches with about 35% slopes. Usually started at around 22mph, but did happen around 26/27 as well. As soon as I eased up on the gas, it would stop and would not continue when I pressed back down.

The shudder is very small, and is not very violent, but definitely noticeable for all in the car. And of course, as soon as we got our mechanic to jump in the car, it would not preform.

I tried doing some research on the T4R forum and found thread on the "Rumble Transmission" but that seems to be happening for folks in higher gears.

Any suggestions? Transmission, engine, or torque converter?

Any help would definitely be appreciated. My dad travels a ton for work so having a working vehicle is imperative.
Thanks!