The outboard lever is your Mechanical Four Wheel Drive (MFWD) control lever. Pull up to engage four wheel drive, push down to take it out. Engage and disengage MFWD when stopped or at low speed or it may growl at you, however, it can be done at speed if you don't mind the noise. When MFWD is engaged you will have four wheel drive and most importantly on hills, will have four wheel braking.
The inboard lever is your differential lock. It locks the left and right rear axles together for maximum traction. Pushing down on the lever engages the differential lock, engage either stopped or at very low speed. The differential lock will stay engaged as long as the rear wheels are slipping, if you continue to slip and regain traction, hold the lever down with your foot so it remains engaged until you are out of trouble.
Bill