That vintage the VIN may have been stamped into the very rear end of either side frame rail. If a sticker was used it may have long since disappeared. That plate today is generally located up front on the side rail and is almost always a sticker of sorts that tend to fade over time.
I have an 83 and it is in a few different places, normally it will be stamped inside the left frame rail, and if not to rusted up you can shine a flashlight in there, EZ also put all of the information on the left side of the upright for the winch and then of course where you found it.
