As I listened to the caller discribe the sound coming from his Ford Explorer chills went down my spine. the exact same thing happened to our 1995 Explorer 3 years ago, and our solution turned out to be more expensive then it needed to be. The symptoms were a loud winning noise and teh car went into a “freewheel”, that is the transmission was not engaged.
After taking the car to 3 experts, including 2 dealers and a transmission shop, we took their advice to get a transmission rebuild. The car was fine for about 3 miles, until I reached the speed of 50 mph.
I took the car back the next day and they did a rebuild of the transfer box and found the problem. It turns out that the fluid circulation in the transfer box gets sucked up through a plastic fitting on the bottom. This was broken!
After that repair the Explorer ran great, and we traded it in for less then that total repair cost us!
Did you get your money back on the misdiagnosed transmission failure?
You do say the problem really was in the transfer case.