Why does the truck growel and shudder when using a gear too high for the rpm of the engine? What is happening when it does this?

You answered your own question

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2021 F150 5.0 Screw

Past:18 F150 2.7, 16 F150 5.0, 14 F150 5.0, 07 F150 5.4, 03 Ranger 4.0

Master Certified Ford Parts / Warranty Admin

For the same reason you do when you attempt to push something heavy while running. If you walk you can push it much easier, your engine is the same, overloading is what its called.

Are you serious? It's the same thing as a 135 lb man carrying his 300 lb bride over the threshold.

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I need to clarify. I know what lugging an engine is. What is the source of the vibration? Starvation of lube oil to the tranny gears? Pistons? Where does the vibration come from?

engine. you are almost forcing a misfire situation

__________________
2021 F150 5.0 Screw

Past:18 F150 2.7, 16 F150 5.0, 14 F150 5.0, 07 F150 5.4, 03 Ranger 4.0

Master Certified Ford Parts / Warranty Admin

Hmm, seems it is a mystery. Will see what I can find out from an engine builder and let ya'll know what I learn.

Engine balance is achieved by crankshaft counterweights and sometimes by an additional balance shaft driven by the timing belt/chain. When the engine runs slower than it was designed for, these systems become less efficient at keeping the engine balanced, since fuel burns at a somewhat constant rate, the space between power strokes becomes more exaggerated.

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2011 Ford Ranger Sport 4x4
4.0L SOHC
5 Speed Manual
MBRP stainless catback
"Air Box Mod" with K&N drop in
GlowShift Oil pressure & Volts in A pillar pod
T-bar crank to level

I changed my search terms and included the word "transmission" in it and this is what came up. Pretty definitive and makes sense.

"Lugging an engine is like hammering the engine parts with every explosion in a cylinder. It racks on the rod bearing/journals, makes the pistons slap the side of the cylinders hard, and if is done enough, could probably break piston rings. It also creates a hammer effect all the way through the drivetrain. There are springs in the friction disk (of the clutch) which takes up some of this hammering, but they will only take so much before they wear out as well. This causes premature wear all the way around. It's not like it will wear out tomorrow, but it will cause wear over time. If you want your car to last a while, you don't want to be doing this. "