A novel polarimetry scheme is proposed to improve the performance of phase-sensitive Surface Plasmon Resonance (SPR) biosensors. The scheme uses s-polarized light, not affected by SPR, as a reference beam, while information on the phase of the p-polarized component is obtained from an analysis of phase-polarization state of light of mixed polarization. We utilize temporal modulation of the beam reflected under SPR by a photo-elastic modulator and show that, under certain birefringent geometry, the signals at the 2nd and 3rd harmonics of modulated frequency can provide ultra-sensitive phase-based response to changes of the refractive index (thickness) of thin films on gold. We also show that the proposed configuration significantly improves detection limit compared to conventional intensity-sensitive SPR, yet enables to maintain wide dynamic range of measurements, which is normally difficult with phase-sensitive SPR schemes. Biosensing applications of the proposed scheme are illustrated in a biological model reaction of avidin - biotin binding on gold.