That's great information, thank you.
Vibrations below about 20 Hertz (cycles per second) aren't audible to most humans.
When a wheel or tire is the source of the vibration, the frequency of the vibration will be around 11 Hertz at 60 mph. Too low to hear or cause a drone or roar.
The driveshaft and engine
are turning around 35 Hertz at the same speed. This is in a range where you can both hear it as roar or drone, and you can feel it.
So this puts the source at the engine, transmission, transfer case or drive shaft.
If you can get the vibration while the truck is sitting and you rev the engine to the same RPM as when the vibration happens, then the source is the engine. But if it only happens when the truck is moving, then that eliminates the engine and torque converter, and anything on the input side of the transmission.
When this happens, does it phase in and out while you're going down the road. In other words does the noise come and go every few seconds?
This is most likely caused by the driveshaft, or something that's causing the drive shaft to be out of round.
Could be bent, could be a bad flange on the differential or the transfer case, could be a u-joint coming apart or installed incorrectly.
If I was was working on this, I would put it up on a hoist and load the suspension (compress the springs) and run it up to 65. Sometimes you can look at the driveshaft and the edges will look fuzzy. This is because it's vibrating. But I also use a tool called a Steelman Chassis Ear. It's got microphones that you can clip onto components. Then the display on the box will show the intensity of the noise or vibration. You switch back and forth between the microphones and it will show you which microphone is picking up the most sound. It's an amazing tool, we call it the magic ears.
Put one microphone on the back of the transfer case, one on the differential, one on the center carrier support, and one on the transmission cross member.
I don't know of another way to diagnose this with tools that are readily available.