vibration
Could be a lot of stuff. The reason you only feel it at certain speeds is because of the harmonics of vibration. The best place to start I think is to get a buddy, or a wife, to sit in the cab and turn the wheel from side to side from center maybe 20 degrees, not lock to lock, while you lay on the ground and watch for any looseness in the steering components. Search the forum and you'll find a lot of likely suspects to inspect. I'm guessing you've checked all the easy stuff like missing weights on a tire.
I had a car that did the same thing I never had any money so I just let it go then I had a blowout and broke down and got new tires. And guess what, It drove fine after that. Could just be that you need new tires. Or tires balanced as the guy above me said.



