vibration while driving around 40 mpg
We need a lot more information. That statement is proper to say to your mechanic/tire shop, but if you’re asking for advice online, you need to give details.
Can you really get 40 mpg from a Pathfinder?
If the vibration clears up at 70 mph, it’s usually a wheel out of balance. If it gets worse, there’s probably damage to a tire or suspension component. Either way, your first recourse is a tire shop.