The car will go into 4wheel-drive high when I pull up on the transfer case handle and the "4wd" lights up on the dash. However, when stopped and I push the transfer case handle forward to come out of 4 wheel-drive the "4wd" light on the dash goes out but the car remains in 4 wheel drive until I drive a couple of hundred feet, and then I hear a "clunk" as the 4 wheel drive disengages. The dealer's mechanic checked out the entire 4WD system and states that all is good with the transfer case at this time but indicates that there is a difference in circumference of the 2 front and 2 rear tires(the rear tires are new and are of the same size as the fronts) which may be the culprit. He says that he has seen this condition before and that I will not damage the transfer case. Can he be correct?