2004 2500HD Manual Transfer Case

I cannot shift into 4HI when moving, get a bad grinding when trying to engage manual transfer case. The technician at the GMC Dealership insists this is no problem. I have had truck for 4 years now and never had this problem. It is a 2004 with 72000 miles on it. It will work fine if you are stopped and shift it. This problem just recently started. It seems that the case will not stay synchronized. Any info appreciated.