Why Differential Standards do not exist in FPGA PlanAhead?
Were you trying to assign those pins as outputs? Xilinx Spartan3 FPGAs have a restriction that the differential standard outputs can be assigned to Banks 0 or 2 only. Only a few combinations of IOSTANDARDS are also possible on those banks. For further details, please refer to Differential I/O Standard Bank Compatibility" section under chapter 10 in UG331.

Not sure why the differential standard is not showing up in the "Create IO Ports" option in PlanAhead. However, if you want to assign it through .ucf, you can use this syntax:

NET  LOC =  | IOSTANDARD = LVDS_25;
NET  LOC =  | IOSTANDARD = LVDS_25;

This example is assuming that you are trying to use 2.5V LVDS IOs. If you are trying to use another IOSTANDARD, please substitute the keyword for the respective standard.