I have some serious vibration at about 40 to 50 MPH only when the accelerator is depressed. As soon as I let off the gas it stops. I have had the tires rotated and balanced and the U joints checked. My Mechanic says he can’t figure it out and to drive it until it breaks.
You can’t always tell if a U joint is good unless you take it out. Your mechanic should know this. When a U-joint breaks, it can tear up the drive shaft and the attached yoke and that becomes a very expensive repair.
He removed the drive shaft and inspected them or so he said.
If he went to the trouble of removing the driveshaft, it wouldn’t have been much more work to replace the u-joints. U-joints are reasonably priced. I am NOT implying that the u-joints were bad, by the way.