I bought this vehicle couple months ago from an auction with water damage, it was up to the bottom door. it had the usual, knocking sound from the transfer case when you select gear to reverse or drive and it would drive smooth after. A week ago I wanted to reverse and it makes this loud buzzing noise, I took a look under and I saw the spot where the drive shaft and transfer case connects vibrates, I guess it tries to switch to reverse and it’s stuck. And just now it happened also when I switched to drive.
what could this be ? There is almost no information and the car has no factory warranty as it has a previous water damage

thank you for your help