Pretty sure '91 classics have the Borg Warner transfer box, you might want to check your viscous coupling. Easy way to tell which transfer box you have is look at Hi/Lo lever, if it only has Hi-N-Lo on it and can only move backward and forward it is BW box. There is no diff lock position on a Borg Warner transfer box which is what I think Wammers meant to say.
Alternatively, get under the Rangie, look around the back of the box there should be a small plate with the box serial number on it. That plate if BW will have red printing on it and confirm it is BW. HTH.