1998 Toyota 4Runner Owners Manual - Page 156

4RUNNER (1998) The rear differential lock system is provided for use only when wheel spinning occurs in a ditch or on a slippery or ragged surface. This differential lock system is effective in case one of the rear wheels is spinning. Before using the rear differential lock system, first shift the front drive control into "L4" to see if it works. If this has no effect, additionally use the rear differential lock system. CAUTION Do not use the rear differential lock in the conditions other than above. Large steering effort and careful cornering control will be required.