Hello;
The best way to do phase difference analysis is by using one of the Simultaneous Multifunction I/O DAQ boards, such as the 6110.
That board has one ADC per channel, meaning that the acquisition is trully simultaneous. If you hook up the voltage waveform to one channel and the current waveform (through a resistor) to another, you can see the phase difference in a simple graph.
Hope this helps.
Filipe