A drive control system for a work vehicle, comprising: a power transmission device having a drive mode switching mechanism that performs drive mode switching between a four-wheel drive mode in which drive power is transmitted to front wheels and rear wheels and a two-wheel drive mode in which drive power is transmitted to only the rear wheels; a drive mode control unit that outputs a drive mode switching request to the drive mode switching mechanism; and a request determination unit that determines a necessity for an increase in drive power on a contact area of the work vehicle and outputs an increase request based on the determination result, the drive mode control unit outputting the drive mode switching request to the drive mode switching mechanism for performing the drive mode switching to the four-wheel drive mode, in response to output of the increase request, characterized in that the power transmission device has a high-speed drive mode switching mechanism to switch to a high-speed drive state in which the front wheels are rotated at a faster ground speed than the rear wheels, that the drive control system further comprises: a steering angle detection unit that detects an over-steering angle at which a steering angle of the front wheels formed as steering wheels is greater than or equal to a predetermined value; and a high-speed detection unit that detects a high-speed drive state, and that a second increase request requesting transition to the high-speed drive state is outputted in priority to the increase request, when the over-steering angle is detected. a power transmission device having a drive mode switching mechanism that performs drive mode switching between a four-wheel drive mode in which drive power is transmitted to front wheels and rear wheels and a two-wheel drive mode in which drive power is transmitted to only the rear wheels; a drive mode control unit that outputs a drive mode switching request to the drive mode switching mechanism; and a request determination unit that determines a necessity for an increase in drive power on a contact area of the work vehicle and outputs an increase request based on the determination result, the drive mode control unit outputting the drive mode switching request to the drive mode switching mechanism for performing the drive mode switching to the four-wheel drive mode, in response to output of the increase request, characterized in that the power transmission device has a high-speed drive mode switching mechanism to switch to a high-speed drive state in which the front wheels are rotated at a faster ground speed than the rear wheels, that the drive control system further comprises: a steering angle detection unit that detects an over-steering angle at which a steering angle of the front wheels formed as steering wheels is greater than or equal to a predetermined value; and a high-speed detection unit that detects a high-speed drive state, and that a second increase request requesting transition to the high-speed drive state is outputted in priority to the increase request, when the over-steering angle is detected. a steering angle detection unit that detects an over-steering angle at which a steering angle of the front wheels formed as steering wheels is greater than or equal to a predetermined value; and a high-speed detection unit that detects a high-speed drive state, and that a second increase request requesting transition to the high-speed drive state is outputted in priority to the increase request, when the over-steering angle is detected.