I had a buddy that had an explorer do the same thing. 60-75 crazy grinding whining noise. as soon as the car dropped below 40mph exactly it went away every time. It started after doing it after his wife was doing like 45-55mph in 4low. Then it was really hard to get it into 4low without it grinding real bad. I thought it was the transfer case, but he said the hell with it and traded it in. so I never found out if that was it....

but does your 4x4 still work smoothly??? low and high.
might be worth a try <img src="/forums/images/graemlins/scared.gif" alt="" />