1993 Jimmy growl/whining noise even in park/digital dash spedo spikes please help

Hi this is the craziest car problem I have ever encountered. My former 1993 Jimmy 4.3 4x4 had a grinding/growl/whine from the front end while in rear wheel drive it sounded very violent. I could push the four high button and it would go away. I talked with a trans specialist and he said replace the vent switch on top of the transfer case. The problem was the switch wore out I could visibly see this when I removed it. As a result the transfer case was not venting vacuum to atmosphere and was trying to slowly engage but it could not mesh together, hence the grinding. Fixed it and it was good.
So I sold this vehicle a couple days ago and they guy I sold it to told me that he has the truck in drive and it won't go anywhere. I looked underneath to see if the driveshaft was busted or fluid leaks, nothing, it looked good.
I started the vehicle in park like normal and it made almost the same whining/ grinding noise instantly really loud. The digital dash spedometer was spiking up while in park. I put it in drive and gave it gas and it would not go. Shifted in and out of 4Hi and 4Lo and the sound went away also got the vehicle to drive fine short distances in the parking lot. Shifted into 2wd would not go anywhere growled horribly finally got it in 4HI and parked it. It did not seem like a slipping trans, trans fluid was full and smelled fine. I teach automotive technology education at the high school level but this is just beyond me. I took it to my transmission/transfercase/diff guy but I need other insight asap please and Thank you. Oh lastly it is like -12 degrees here if that has anything to do with it.

Found out it was the transfercase. Got a junkyard one for $150.