So I just noticed this the other day, as I don't really do many highway trips during the warmer months in the Rover. I was going to the airport, and about 20 miles on the highway into the drive, I noticed a small vibration. Almost like a tire out of balance but not as bad. It was only really above 65MPH. Got to the airport, checked tires and all and no issues. On the way home a few days later, same thing. No vibration for the first 3/4 of the drive, then started up towards home. Didn't matter if on the gas or off, and didn't change when I quickly put it in nuetral to take the trans and such out of the equation.
It really has me stumped... what would cause a slight vibration when the truck is "warmed up". Any clue on this one? Really not sure where to start looking. It is not major, but it is coming into ski season where I make many longer trips with it.
06 HSE with HD, 105K, Nokian snow tires all at 38psi

 

Mine does the same thing. Usually around 75-80 and it reminds me to slow down! It's difficult to notice but you can see it in the mirrors.

 

Try rotate and balance tires, check suspension bushings, wheel bearings ,sounds like something that takes time to warm up is causing the vibration IMO

Have you serviced the diffs? , transfer case? Have leaks?
Could have simply lost a wheel weight