It is considered bad design practice to set your pin-out in an RTL project.
Ports should be defined as differential in HDL or within a Vivado Pin Planning project.
As a result, the "Make Diff Pair" option has been hidden in RTL projects for Vivado 2012.4 and later versions.
If the design needs to be modified outside of the HDL/Pin Planning project, the following TCL command can be used to create the differential port:
make_diff_pair_ports port_p port_n