99 Tahoe transfer case

My Wife's Tahoe has been acting up lately. Its making a weird noise at lower speeds almost like a grinding, and she says that one day it went in to 4 wheel drive on its own. The car place she took it too didnt know what to tell her, they drove it, put it on the lift, its not leaking, but still making the noise. They said to replace the transfer case, but im not sure I agree. It has 120,000 miles, and the 4 wheel drive hardly gets used. Its a 4 door, push button 4 wheel drive. I would love to help her and tear into it, but im deployed over seas. Thanks for the help.