1999 Cadillac Escalade shift into overdrive vibration

When accelerating on to freeway and as tranny shifts thru all gears smoothly, upon shifting into O/D, the vehicle goes into a noticeable vibration, 65mph and then we have to back our speed down to 50mph and vibration goes away we can then resume to normal freeway speed 75 80 mph with no issues. I have checked motor and tranny mounts. Also checked all driveline and front u joints. Interested in your thoughts! Thanks for your time