Low ratio is for high torque applications and also when you need the front and back axle locked, think slow offroading where you need control over speed.
To engage >
Stop, but the truck in N, leave for a second for the gearbox to spin down, pull the transfer case lever up in to N then gently up again into 4wd low, it might not go in so move the steering from lock to lock and it'll snick up. Put the truck in 'd' and see if you move off - if you hear grinding, your not in low, so back to 'n' wait for the box to spin down and try again.
When your in low, it's worth putting the auto box in either 1 or 2 to limit speed and increase engine braking. be aware of a harsh gearchange if you move from 1 > 2 in the auto box whilst in low though.
When in low ratio treat the pedals like your standing on egg shells, just gently gently on everything. There's double the torque travelling through your transmission and breaking things can happen.
Don't use 4 low on tarmac.
