A differential assembly for a heavy vehicle, said differential assembly comprising a rotating support element, a differential, and torque-selector means, said rotating support element being carried, in such a way that it can rotate freely, by a casing and being connectable to a drive shaft of said vehicle, said rotating support element defining an internal volume configured to house said differential and said torque-selector means, said differential being configured to divide a torque at input into a pair of shafts (5, 6) of said vehicle, said torque-selector means connecting said rotating support element to said differential according to a plurality of different transmission ratios, wherein said torque-selector means comprise a selector shaft, a ring gear, a planetary gear train, the sun gear of which is said selector shaft and comprising a plurality of planet gears, which mesh between said selector shaft and said ring gear, said differential assembly being characterized in that said torque-selector means comprise a pair of second gears (23', 23"), said planetary gear train comprising a pair of gear-train carriers (33', 33") carried by said planet gears, each meshing, respectively, with one of said second gears (23', 23"), said first gear is connected to said ring gear, to said pair of second gears (23', 23"), and said ring gear comprises a cylindrical bushing housed within said volume and configured to cooperate with an outer surface thereof in sliding manner with the inner surface of said rotating support element and defining an inner toothing that radially extends radially inner on the opposite side with respect to said support element, one of said second gears connecting said first gear to said differential and the other of said second gears (23") connecting said first gear to said rotating support element, said planetary gear train and said pair of second gears (23, 23') modifying the torque/velocity between said rotating support and said differential according to the position of said selector shaft with respect to said casing, each of said second gears (23', 23") comprises a toothed ring which constantly externally meshes with said inner toothing of said ring gear and can selectively internally mesh with said rotating support element, and a toothed bushing, which is slidingly carried internally by said toothed ring, and on the inside constantly meshes with a respective gear-train carrier (33', 33"), wherein one of said toothed rings and one of said toothed bushings can selectively mesh with said differential, the other toothed ring and the other toothed bushing can selectively internally mesh with said rotating support element. said rotating support element being carried, in such a way that it can rotate freely, by a casing and being connectable to a drive shaft of said vehicle, said rotating support element defining an internal volume configured to house said differential and said torque-selector means, said differential being configured to divide a torque at input into a pair of shafts (5, 6) of said vehicle, said torque-selector means connecting said rotating support element to said differential according to a plurality of different transmission ratios, wherein said torque-selector means comprise a selector shaft, a ring gear, a planetary gear train, the sun gear of which is said selector shaft and comprising a plurality of planet gears, which mesh between said selector shaft and said ring gear, said differential assembly being characterized in that said torque-selector means comprise a pair of second gears (23', 23"), said planetary gear train comprising a pair of gear-train carriers (33', 33") carried by said planet gears, each meshing, respectively, with one of said second gears (23', 23"), said first gear is connected to said ring gear, to said pair of second gears (23', 23"), and said ring gear comprises a cylindrical bushing housed within said volume and configured to cooperate with an outer surface thereof in sliding manner with the inner surface of said rotating support element and defining an inner toothing that radially extends radially inner on the opposite side with respect to said support element, one of said second gears connecting said first gear to said differential and the other of said second gears (23") connecting said first gear to said rotating support element, said planetary gear train and said pair of second gears (23, 23') modifying the torque/velocity between said rotating support and said differential according to the position of said selector shaft with respect to said casing, each of said second gears (23', 23") comprises a toothed ring which constantly externally meshes with said inner toothing of said ring gear and can selectively internally mesh with said rotating support element, and a toothed bushing, which is slidingly carried internally by said toothed ring, and on the inside constantly meshes with a respective gear-train carrier (33', 33"), wherein one of said toothed rings and one of said toothed bushings can selectively mesh with said differential, the other toothed ring and the other toothed bushing can selectively internally mesh with said rotating support element.