I had same issue with my QX4. As soon as I hit 50mph, I would get that groaning vibration through the floor. It was less noticeable after 65mph-70mph. Breaking, coasting, hard lefts and rights... no change. I then switched from 2WD to "auto" and it went away as long as I was accelerating. If I let off the gas, it came back. I got under it and checked driveshafts for play, front one had some play in it. Replaced u-joints, did not solve issue. Replaced trans mount because it was shot... no change. When I replaced both front CV axles and wheel bearings, it was smooth as glass again... I know I am a year late but.... for anyone else having the same problem..