One problem related to differential unit has been reported for the 2003 GMC C2500. The most recently reported issues are listed below.
I have a 2003 GMC sierra 2500hd. The four wheel drive works intermittently. According to GMC document id#1400441 there is no fix for this problem now. However, they are working on the problem and as soon as a fix is available they will make the repairs in the trucks. This problem applies to 2003 and 2004 k series. These trucks are not only used personal but in some cases, also in emergency rescue and fire services where the 4 wheel drive may be needed to gain access to patients. I can be contacted for further details.