I have a similar problem with a '99 disco II (117k miles) which I just purchased for my son. I took the vehicle to my dealership (after installing new tires) and they told me that the transfer case needed to be rebuilt. I am not buying it just yet and am going to try adding Cermet to the transfer case and front & rear axles. The vibration seems to come in harmonic waves between 60 and 75 mph and varies depending on road surface and at times seems to go away altogether. I will come back and post if this works

I do have a question for the wise mechanics out there. The rear axle has a weighted harmonic balancer with a ground cable which on this vehicle is broken. Does anyone know the importance of this ground?

Thanks all,
Peter