That is correct, but the VIN tag is still located in the truck. People just drill them out and put one from their truck on it instead. Therefor its your VIN from your driver but installed on a different cab. Keep in mind this isnt legal and in most states is a felony. I have done it several times in my youth with cars and trucks for various reasons. But now I dont mess with it as I dont need the trouble.
Around here the junkyards cant sell you a complete cab. They must cut it in half somewhere. Like for example with a blazer they cut the bed and roof off and separate it, so you cant just bolt it on and go. Thats how they get around selling the cab, as its for restoring a damaged one and not to be drove. Funny thing is, they will cut it in half, and you can buy both halves.
Your best bet, would be either fixing it, or buying a privately owned Blazer and doing the swap that way, the chances of you actually getting caught with the VIN tags switched is about less than nil, but it can and does happen.