1998 GMC Suburban Owner's Manual - Page 82

Power Door Locks Press the side the power of door lock switch marked LOCK on either front door to lock all the doors once. at Press the ribbed side the of switch tounlock all the doors at once. On four-door models,the manual switch on each rear door works only that door's lock. won't lock (or It unlock) all of the doors -- that's a safety feature.