Transfer Case
WARNING: This page does not describe the selected car, but rather 28 other vehicles, including the 1988 GMC Vandura Special, 1988 GMC Vandura, 1988 GMC Value Van, 1988 GMC Suburban, and 1988 GMC Safari. However, it is still accessible from the selected car via links, so may be relevant.
Remove filler plug. Check oil level. If level is not up to filler plug opening, add lubricant until it is.