X5 shakes/rattles when keeping speed on uphill

[x5turtle]

It happened to me while I was driving to Grand Canyon with my family last weekend. Car was driving fine when going up the Canyon, and when I drive to Las Vegas after the Canyon, the car shakes or rattles when I try to maintain speed on slight uphill at 80mph. If i let go of gas, X5 feels fine, but when you hit the gas to maintain speed, it rattles again.

However, after stopping on the side of the road for few minutes, the rattle stop, and it comes back after a little while. I did it a few times and I finally slow down to 65mph, then the problem doesn't show up as much.

I also put it in neutral and rev the engine to 3k rpm while the car is traveling at about 70mph, there is no rattle, but when I put it back into drive and try to maintain speed, it rattles. Again, it happens less at 65 mph.

It doesn't happen on my 20miles drive to work on the freeway, but it just started to happen to me on long drives. Anyone ever experienced this?

My X5 is about 140K, I am first owner, maybe the transmission is going out on me?? Any thoughts or inputs will be very much appreciated! I don't want to sell the car.

Please Help!