A differential gear mechanism of the type including a gear case defining axis of rotation and a gear chamber; differential gear means disposed in said gear chamber including at least one input gear and first and second output gears; means operable to prevent rotation of said first output gear relative to said gear case; actuating means for actuating said rotation prevention means in response to an input signal; said actuating means comprising a ramp plate disposed about a portion of said gear case adjacent said second output gear, and somewhat radially outward therefrom, said ramp plate being disposed, in an unactuated condition (FIG.3) of said actuating means, to rotate with said gear case, said actuating means further including an actuator (33,57) operable when said actuating means is in an actuated condition (FIG. 1), to cause said ramp plate to rotate at a speed of rotation different than that of said gear case; characterized by: (a) a collar member fixed to rotate with said gear case, but free to move axially relative to said gear case, and cooperating with said first output gear to define lock members (41,43) operable, when said collar member is in a locked position (FIGS. 1 and 2), to prevent rotation of said first output gear relative to said gear case; (b) biasing means disposed axially between said collar member and said gear case, tending to bias said collar member, from said locked position (FIG. 1), in a direction toward said second output gear, to an unlocked position (FIG. 3); and (c) a plurality of elongated actuation members, each having a first end in engagement with said collar member, and a second end in engagement with said ramp plate whereby, when said actuating means is in said actuated condition, and said ramp plate rotates relative to said gear case, said elongated actuation members ramp-up on said ramp plate and move in an axial direction toward said first output gear, moving said collar member from said unlocked position (FIGS. 3, 4 and 5) to said locked position (FIG. 1), in opposition to the force of said biasing means. (a) a collar member fixed to rotate with said gear case, but free to move axially relative to said gear case, and cooperating with said first output gear to define lock members (41,43) operable, when said collar member is in a locked position (FIGS. 1 and 2), to prevent rotation of said first output gear relative to said gear case; (b) biasing means disposed axially between said collar member and said gear case, tending to bias said collar member, from said locked position (FIG. 1), in a direction toward said second output gear, to an unlocked position (FIG. 3); and (c) a plurality of elongated actuation members, each having a first end in engagement with said collar member, and a second end in engagement with said ramp plate whereby, when said actuating means is in said actuated condition, and said ramp plate rotates relative to said gear case, said elongated actuation members ramp-up on said ramp plate and move in an axial direction toward said first output gear, moving said collar member from said unlocked position (FIGS. 3, 4 and 5) to said locked position (FIG. 1), in opposition to the force of said biasing means.