Could also be the bearings , the seller may not have noticed the noise out of diff lock as there is usually less load within the transfer case , but if there is too much clearance the gears will be out of mesh especially under load .
Worst case scenario is the centre diff shims / other bearings are U/S and have chewed through the gears in which case probably not worth it. Maybe ask if the seller can take a pic with the lower cover off ( only 10 bolts ) of the intermediate and input gears so you can see if they are salvageable.

