1996 GMC Jimmy transfer case

Have 96 gmc jimmy and heard some rattles upon accelerating and when accelerated faster the rattles went away and it was a bit rough driving. I though I had some transmission problem so took truck to shop and was told my wife drove the truck in 4 wheel drive because she forgot to put it into 2 wheel that the transfer case not good and would cost alot to fix and since 2 wheel works great we can drive it without ever using 4 wheel but that we would hear noises because of the transfer case. Why would I hear rattles etc if in 2 wheel drive? Can you advise. Mechanic said can drive but wont ever have access to 4 wheek.