To assist with traction, Toyota has offered diff locks on 2wd trucks since the introduction of the TRD package in 1998.
Many owners desire better off road ability with their 2we trucks and don't want or need 4wd.
Do you have a diff lock button on the dash? I don't know what model/yr truck you have but most trucks with a diff lock, also known as an electronic locking differential, have a button on the left side of the dash.
Later model yr c/c pre runner trucks didn't always come with this option, it depended on which packages you had that determined the availability.