An axle assembly comprising: a housing assembly that receives a differential assembly; an input shaft that is rotatable about a first axis and is at least partially received in the housing assembly; and an interaxle differential unit, characterized in that the axle assembly further comprises: a gear reduction unit that is disposed in the housing assembly and is operatively connected to the input shaft, wherein the gear reduction unit includes a planetary gear set that has a sun gear (260, 600, 600') that is rotatable about the first axis, a planet carrier that rotatably supports a planet pinion, and a ring gear that is fixedly positioned with respect to the housing assembly; and a first coupling (172', 604, 604') that is moveable between a first position in which the first coupling (172', 604, 604') couples the input shaft to the sun gear (260, 600, 600') such that the sun gear (260, 600, 600') and input shaft are rotatable together about the first axis and a second position in which the first coupling (172', 604, 604') couples the input shaft to the planet carrier such that the planet carrier and the input shaft are rotatable together about the first axis; and the interaxle differential unit is received in a case that is at least partially defined by the planet carrier and that operatively connects the planetary gear set to the differential assembly and an output shaft (76, 500). a housing assembly that receives a differential assembly; an input shaft that is rotatable about a first axis and is at least partially received in the housing assembly; and an interaxle differential unit, characterized in that the axle assembly further comprises: a gear reduction unit that is disposed in the housing assembly and is operatively connected to the input shaft, wherein the gear reduction unit includes a planetary gear set that has a sun gear (260, 600, 600') that is rotatable about the first axis, a planet carrier that rotatably supports a planet pinion, and a ring gear that is fixedly positioned with respect to the housing assembly; and a first coupling (172', 604, 604') that is moveable between a first position in which the first coupling (172', 604, 604') couples the input shaft to the sun gear (260, 600, 600') such that the sun gear (260, 600, 600') and input shaft are rotatable together about the first axis and a second position in which the first coupling (172', 604, 604') couples the input shaft to the planet carrier such that the planet carrier and the input shaft are rotatable together about the first axis; and the interaxle differential unit is received in a case that is at least partially defined by the planet carrier and that operatively connects the planetary gear set to the differential assembly and an output shaft (76, 500). a gear reduction unit that is disposed in the housing assembly and is operatively connected to the input shaft, wherein the gear reduction unit includes a planetary gear set that has a sun gear (260, 600, 600') that is rotatable about the first axis, a planet carrier that rotatably supports a planet pinion, and a ring gear that is fixedly positioned with respect to the housing assembly; and a first coupling (172', 604, 604') that is moveable between a first position in which the first coupling (172', 604, 604') couples the input shaft to the sun gear (260, 600, 600') such that the sun gear (260, 600, 600') and input shaft are rotatable together about the first axis and a second position in which the first coupling (172', 604, 604') couples the input shaft to the planet carrier such that the planet carrier and the input shaft are rotatable together about the first axis; and the interaxle differential unit is received in a case that is at least partially defined by the planet carrier and that operatively connects the planetary gear set to the differential assembly and an output shaft (76, 500).