High Speed Rumble

Did you do the brake tap test ?

 

What you describe as others have said is often due to drive shaft/carrier bearings/ u joints. However since it’s been there since you got the truck I’m kind of wondering if it has to do with the tires themselves. You’ve got 50000 miles on the tires, must be getting pretty close to replacement time or maybe already here. When you replace them get a different set than what you have. Be interesting to see if any difference..
Actually the more I think about it if it was something moving, metal and mechanical then in 50000 miles almost 100% it would get worse and probably fail over that many miles. Since no worse I’m going with the tires. Or the noise you hear is normal and you are just extra sensitive to it.

 

Gearcruncher, I did do the brake tap test. In fact, I pretty much did everything I could do on your list. The tests all came out OK. If you are interested, my notes are in the attached file). Again, thank you for the very complete vibration test list. I'm keeping a copy. I expect it to come in handy as the truck ages.

Sprig, I'm kind of leaning that way myself. The tires are original and the tech tells me they are soft and show uneven wear on the rear. He recommended I over inflate them to 35 psi and get some beefier tires for the next set. So far nothing has broken and the vibration hasn't gotten worse. You might be right and I'm just being too sensitive. It is, after all, a truck not a car.

Thank you guys for the help.