Trailer center axle does not move to open four-wheel drive in order to go a what is bad

First of all, the differential is able to fix either half of the axle, keep the other half of the axle rotation, whether it is the center differential or the rear differential, but the rear differential rely on the bevel gears to complete the shaft steering function, the central differential axle direction is basically the same, resulting in the internal structure of the difference. Since this is said, the inter-axle differential can certainly complete the front and rear axle either fixed and only rotate the other 1 axle function