1998 Ford Explorer transfer case

While driving in 2WD the truck started to shake very bad and the, what I think was the drive shaft clanking. I pulled off the road came to a stop, proceeded to go forward and heard the same clanking noise coming from the drive train. I moved the 4WD selector to low and then to high and back to auto and tried to move the truck at the same time but the clanking continued. There is some kind of binding in the forward drive train I think transfer case. I now have it on jack stands, forward drive shaft out and looks good. I have spun the front wheels by hand and there is no noise or binding going on. I removed the shift motor and was able to turn the shift shaft didn't notice anything wrong. I dropped the transfer case oil and didn't find anything wrong. Do you have any clues as to what is wrong. Thank you