Hi all! My Sporty has developed a strange problem and I am hoping someone can help!

When driving, once I reach about 45 to 50 MPH, I start to feel a vibration in the drivetrain, and it gets worse if I go faster. It does it when power is being applied to the wheels to move forward, so if I'm cruising or accelerating it vibrates, but if I let go of the gas and allow the car to engine brake, the noise and vibration stop.

Not only that, but if I accelerate to 50 or 60 MPH and then put the car in neutral, the noise and vibration will continue. HOWEVER, as soon as I put it back in gear, so that now I am starting to engine brake, the noise and vibration stop. Then, if I put it in neutral again, the noise will be gone.

So, the noise is tied to what speed I'm going, not the engine RPM. Engine RPM has no bearing on it, just speed. The other factor is whether the drivetrain is loaded up or dragging / engine braking. Noise only happens when the drivetrain is loaded up.

It doesn't matter what gear I'm in, just the speed. I can go the same speed in 5th or 4th gear and the noise is the same, or even if I'm in neutral the noise is the same, but noise goes away if I engine brake then put it in neutral. As soon as I engine brake even a tiny bit, bam the noise is gone and stays gone until I load the drivetrain (accelerate).

Any ideas? My first thought was the rear differential going bad, or maybe out of balance somehow, or maybe the driveshaft out of balance?

Also the transmission, transfer case, and differential all have enough gear oil in them, I checked that. The gear oil in the rear diff looks relatively clean to me, but has a slightly reddish-brown look to it, is that normal?

Any help would be appreciated! I'm about ready to take it to a shop if I can't figure it out very soon.