An effective long-haul self-mixing interference effect has been observed in a thin-slice LiNdP₄O₁₂ (LNP) laser due to Doppler-shifted optical feedback from a distant target. The narrow spectral linewidth of the LNP laser, which was evaluated to be 16 kHz by heterodyne measurements, led to successful self-mixing laser Doppler velocimetry and vibrometry of targets placed 2.5 km away from the laser through single-mode optical fiber access.