The Expansion Differential Connectors header (J5), on the ML401/402/403, pinout is given in Table 9 of UG080:

The pins of this header connect to the FPGA I/O and can be used as independent single-ended nets as well as differential signaling, with one exception. Differential pair AD19-AC19 are lower capacitance clock pins that connect to Clock Capable I/Os. These pins do not support LVDS outputs, and become regular user I/Os when not needed for clocks.
When considering how to use the J5 header, it is important to realize that this differential pair does not support LVDS outputs.