Transfer Case Problems?

[angminnie]

Hello. We have a 2005 Silverado 2500HD Diesel 4x4. Recently after using the 4-wheel drive (in soft sand) we have noticed an unusual bang/clunking noise when taking the foot off the gas and slow speeds (20-30 mph). This is consistent at every takeoff. A friend mentioned a possible problem w/ the u-joint but a mechanic looked at it and said the u-joint was fine, but that it could be the transfer case and it would cost $2800 to re-build.