Vibrations can do all sorts of weird things for weird reasons so they can come and go for odd reasons. At 1500rpm & 45-55mph, for example, there is a specific state of vibration that your whole drive train is in that is different from what it is at other times. Cars don't feel like it but getting them to move is a crazy thing - explosions, intense heat, heavy chunks of metal churning around. Perception is also tricky, so if you hang out long enough you'll see lots of people who come here with one picture of things in their minds and then when asked questions, they go back and "re-perceive," come out with a different picture. Anyway, yada yada.
When I looked at your description, I saw "rumble strip" and 45-55mph or so, and a change when you get on or off the gas. That made me think of a torque converter clutch (TCC) lockup problem. This kind of thing is often described as "shudder." It shouldn't interfere with taking a road trip - but its a transmission related thing so its not good.
You could instead hope for something like a CV joint issue which can also come and go as a vibration issue related to acceleration and speed of car. I would cross the tires off, but certainly not CV joints.