Mine also happens over that 40mph. Goes to about 60mph. Then the road noise kind of blurs it out. Goes away when quickly letting off the gas. Not vibration though. Vibration, I would replace the rear joints. Those are typically the issue with vibrations.
Other than possible tires.
Joints vibration might go away when letting off the gas. Tires won't.

 

Thank you. Will make a visit to another mechanic Monday, will see what he is going to say. At least the repair shop yesterday said it is safe to drive.

 

I also believe it's safe. Changing the joints because of the vibration wouldn't be a bad idea unless the problem doesn't go away when you let off the gas. I only have noise myself. It goes away so I'm assuming it's different pressures on the ring and pinion gears causing the issue. Plus since I changed fluids and it was almost 90% better, I knew I was in the right spot. I think my is just gear noise and sometime when I feel like it, I'll get some noise reduction additive to shoot in there once and see what it does. Mine probably just needs a set of gears. I'll wait until the wife rips them out with her driving habits first.