If you read further down in the Wiki page, the 3500's did not change to 16" until 2006 and later..... the message earlier in the in the dialog from 2002 is related to 2500's only. Depending on the production date in 2006 may come into effect of why yours has 15" From what I have seen over the years, most 3500 have been stock at 15"
PM Sailquik (aka Roger) he will run you a build sheet for the van. He's super friendly (Roger) and has provided build sheets to many of the forum contributors, including me. Find one of his posts and PM him, with your vin, and a private email address and I'm sure he will respond. Derek