One problem related to radiator has been reported for the 2006 GMC C4500. The most recently reported issues are listed below.
Pulling a loaded trailer up a steep grade on a major highway. Speed was approx. 40mph. Driver hear a "clunk" noise then the low coolant light came on and then the engine temp started to rise. Pulled over and there was coolant leaking from the front of the truck. Upon further inspection the bottom radiator hose had been cut from the fan that exploded and the fan also tore the fan shroud open too. Summary- fan exploded, which ruined the fan shroud and the lower radiator hose.